On Sat, Jan 12, 2019 at 12:16 PM Christophe Leroy
<[email protected]> wrote:
>
> In kernel/cputable.c, explicitly use memcpy() in order
> to allow GCC to replace it with __memcpy() when KASAN is
> selected.
>
> Since commit 400c47d81ca38 ("powerpc32: memset: only use dcbz once cache is
> enabled"), memset() can be used before activation of the cache,
> so no need to use memset_io() for zeroing the BSS.
>
> Signed-off-by: Christophe Leroy <[email protected]>
> ---
> arch/powerpc/kernel/cputable.c | 4 ++--
> arch/powerpc/kernel/setup_32.c | 6 ++----
> 2 files changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/arch/powerpc/kernel/cputable.c
b/arch/powerpc/kernel/cputable.c
> index 1eab54bc6ee9..84814c8d1bcb 100644
> --- a/arch/powerpc/kernel/cputable.c
> +++ b/arch/powerpc/kernel/cputable.c
> @@ -2147,7 +2147,7 @@ void __init set_cur_cpu_spec(struct cpu_spec *s)
> struct cpu_spec *t = &the_cpu_spec;
>
> t = PTRRELOC(t);
> - *t = *s;
> + memcpy(t, s, sizeof(*t));
Hi Christophe,
I understand why you are doing this, but this looks a bit fragile and
non-scalable. This may not work with the next version of compiler,
just different than yours version of compiler, clang, etc.
Does using -ffreestanding and/or -fno-builtin-memcpy (-memset) help?
If it helps, perhaps it makes sense to add these flags to
KASAN_SANITIZE := n files.
> *PTRRELOC(&cur_cpu_spec) = &the_cpu_spec;
> }
> @@ -2162,7 +2162,7 @@ static struct cpu_spec * __init setup_cpu_spec(unsigned long offset,
> old = *t;
>
> /* Copy everything, then do fixups */
> - *t = *s;
> + memcpy(t, s, sizeof(*t));
>
> /*
> * If we are overriding a previous value derived from the real
> diff --git a/arch/powerpc/kernel/setup_32.c b/arch/powerpc/kernel/setup_32.c
> index 947f904688b0..5e761eb16a6d 100644
> --- a/arch/powerpc/kernel/setup_32.c
> +++ b/arch/powerpc/kernel/setup_32.c
> @@ -73,10 +73,8 @@ notrace unsigned long __init early_init(unsigned long dt_ptr)
> {
> unsigned long offset = reloc_offset();
>
> - /* First zero the BSS -- use memset_io, some platforms don't have
> - * caches on yet */
> - memset_io((void __iomem *)PTRRELOC(&__bss_start), 0,
> - __bss_stop - __bss_start);
> + /* First zero the BSS */
> + memset(PTRRELOC(&__bss_start), 0, __bss_stop - __bss_start);
>
> /*
> * Identify the CPU type and fix up code sections
> --
> 2.13.3
>
On 01/14/2019 09:34 AM, Dmitry Vyukov wrote:
> On Sat, Jan 12, 2019 at 12:16 PM Christophe Leroy
> <[email protected]> wrote:
> >
> > In kernel/cputable.c, explicitly use memcpy() in order
> > to allow GCC to replace it with __memcpy() when KASAN is
> > selected.
> >
> > Since commit 400c47d81ca38 ("powerpc32: memset: only use dcbz once cache is
> > enabled"), memset() can be used before activation of the cache,
> > so no need to use memset_io() for zeroing the BSS.
> >
> > Signed-off-by: Christophe Leroy <[email protected]>
> > ---
> > arch/powerpc/kernel/cputable.c | 4 ++--
> > arch/powerpc/kernel/setup_32.c | 6 ++----
> > 2 files changed, 4 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/powerpc/kernel/cputable.c
> b/arch/powerpc/kernel/cputable.c
> > index 1eab54bc6ee9..84814c8d1bcb 100644
> > --- a/arch/powerpc/kernel/cputable.c
> > +++ b/arch/powerpc/kernel/cputable.c
> > @@ -2147,7 +2147,7 @@ void __init set_cur_cpu_spec(struct cpu_spec *s)
> > struct cpu_spec *t = &the_cpu_spec;
> >
> > t = PTRRELOC(t);
> > - *t = *s;
> > + memcpy(t, s, sizeof(*t));
>
> Hi Christophe,
>
> I understand why you are doing this, but this looks a bit fragile and
> non-scalable. This may not work with the next version of compiler,
> just different than yours version of compiler, clang, etc.
My felling would be that this change makes it more solid.
My understanding is that when you do *t = *s, the compiler can use
whatever way it wants to do the copy.
When you do memcpy(), you ensure it will do it that way and not another
way, don't you ?
My problem is that when using *t = *s, the function set_cur_cpu_spec()
always calls memcpy(), not taking into account the following define
which is in arch/powerpc/include/asm/string.h (other arches do the same):
#if defined(CONFIG_KASAN) && !defined(__SANITIZE_ADDRESS__)
/*
* For files that are not instrumented (e.g. mm/slub.c) we
* should use not instrumented version of mem* functions.
*/
#define memcpy(dst, src, len) __memcpy(dst, src, len)
#define memmove(dst, src, len) __memmove(dst, src, len)
#define memset(s, c, n) __memset(s, c, n)
#endif
void __init set_cur_cpu_spec(struct cpu_spec *s)
{
struct cpu_spec *t = &the_cpu_spec;
t = PTRRELOC(t);
*t = *s;
*PTRRELOC(&cur_cpu_spec) = &the_cpu_spec;
}
00000000 <set_cur_cpu_spec>:
0: 94 21 ff f0 stwu r1,-16(r1)
4: 7c 08 02 a6 mflr r0
8: bf c1 00 08 stmw r30,8(r1)
c: 3f e0 00 00 lis r31,0
e: R_PPC_ADDR16_HA .data..read_mostly
10: 3b ff 00 00 addi r31,r31,0
12: R_PPC_ADDR16_LO .data..read_mostly
14: 7c 7e 1b 78 mr r30,r3
18: 7f e3 fb 78 mr r3,r31
1c: 90 01 00 14 stw r0,20(r1)
20: 48 00 00 01 bl 20 <set_cur_cpu_spec+0x20>
20: R_PPC_REL24 add_reloc_offset
24: 7f c4 f3 78 mr r4,r30
28: 38 a0 00 58 li r5,88
2c: 48 00 00 01 bl 2c <set_cur_cpu_spec+0x2c>
2c: R_PPC_REL24 memcpy
30: 38 7f 00 58 addi r3,r31,88
34: 48 00 00 01 bl 34 <set_cur_cpu_spec+0x34>
34: R_PPC_REL24 add_reloc_offset
38: 93 e3 00 00 stw r31,0(r3)
3c: 80 01 00 14 lwz r0,20(r1)
40: bb c1 00 08 lmw r30,8(r1)
44: 7c 08 03 a6 mtlr r0
48: 38 21 00 10 addi r1,r1,16
4c: 4e 80 00 20 blr
When replacing *t = *s by memcpy(t, s, sizeof(*t)), GCC replace it by
__memcpy() as expected.
>
> Does using -ffreestanding and/or -fno-builtin-memcpy (-memset) help?
No it doesn't and to be honest I can't see how it would. My
understanding is that it could be even worse because it would mean
adding calls to memcpy() also in all trivial places where GCC does the
copy itself by default.
Do you see any alternative ?
Christophe
> If it helps, perhaps it makes sense to add these flags to
> KASAN_SANITIZE := n files.
>
>
>> *PTRRELOC(&cur_cpu_spec) = &the_cpu_spec;
>> }
>> @@ -2162,7 +2162,7 @@ static struct cpu_spec * __init setup_cpu_spec(unsigned long offset,
>> old = *t;
>>
>> /* Copy everything, then do fixups */
>> - *t = *s;
>> + memcpy(t, s, sizeof(*t));
>>
>> /*
>> * If we are overriding a previous value derived from the real
>> diff --git a/arch/powerpc/kernel/setup_32.c b/arch/powerpc/kernel/setup_32.c
>> index 947f904688b0..5e761eb16a6d 100644
>> --- a/arch/powerpc/kernel/setup_32.c
>> +++ b/arch/powerpc/kernel/setup_32.c
>> @@ -73,10 +73,8 @@ notrace unsigned long __init early_init(unsigned long dt_ptr)
>> {
>> unsigned long offset = reloc_offset();
>>
>> - /* First zero the BSS -- use memset_io, some platforms don't have
>> - * caches on yet */
>> - memset_io((void __iomem *)PTRRELOC(&__bss_start), 0,
>> - __bss_stop - __bss_start);
>> + /* First zero the BSS */
>> + memset(PTRRELOC(&__bss_start), 0, __bss_stop - __bss_start);
>>
>> /*
>> * Identify the CPU type and fix up code sections
>> --
>> 2.13.3
>>
On Tue, Jan 15, 2019 at 8:27 AM Christophe Leroy
<[email protected]> wrote:
>
>
>
> On 01/14/2019 09:34 AM, Dmitry Vyukov wrote:
> > On Sat, Jan 12, 2019 at 12:16 PM Christophe Leroy
> > <[email protected]> wrote:
> > >
> > > In kernel/cputable.c, explicitly use memcpy() in order
> > > to allow GCC to replace it with __memcpy() when KASAN is
> > > selected.
> > >
> > > Since commit 400c47d81ca38 ("powerpc32: memset: only use dcbz once cache is
> > > enabled"), memset() can be used before activation of the cache,
> > > so no need to use memset_io() for zeroing the BSS.
> > >
> > > Signed-off-by: Christophe Leroy <[email protected]>
> > > ---
> > > arch/powerpc/kernel/cputable.c | 4 ++--
> > > arch/powerpc/kernel/setup_32.c | 6 ++----
> > > 2 files changed, 4 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/arch/powerpc/kernel/cputable.c
> > b/arch/powerpc/kernel/cputable.c
> > > index 1eab54bc6ee9..84814c8d1bcb 100644
> > > --- a/arch/powerpc/kernel/cputable.c
> > > +++ b/arch/powerpc/kernel/cputable.c
> > > @@ -2147,7 +2147,7 @@ void __init set_cur_cpu_spec(struct cpu_spec *s)
> > > struct cpu_spec *t = &the_cpu_spec;
> > >
> > > t = PTRRELOC(t);
> > > - *t = *s;
> > > + memcpy(t, s, sizeof(*t));
> >
> > Hi Christophe,
> >
> > I understand why you are doing this, but this looks a bit fragile and
> > non-scalable. This may not work with the next version of compiler,
> > just different than yours version of compiler, clang, etc.
>
> My felling would be that this change makes it more solid.
>
> My understanding is that when you do *t = *s, the compiler can use
> whatever way it wants to do the copy.
> When you do memcpy(), you ensure it will do it that way and not another
> way, don't you ?
It makes this single line more deterministic wrt code-gen (though,
strictly saying compiler can turn memcpy back into inlines
instructions, it knows memcpy semantics anyway).
But the problem I meant is that the set of places that are subject to
this problem is not deterministic. So if we go with this solution,
after this change it's in the status "works on your machine" and we
either need to commit to not using struct copies and zeroing
throughout kernel code or potentially have a long tail of other
similar cases, and since they can be triggered by another compiler
version, we may need to backport these changes to previous releases
too. Whereas if we would go with compiler flags, it would prevent the
problem in all current and future places and with other past/future
versions of compilers.
> My problem is that when using *t = *s, the function set_cur_cpu_spec()
> always calls memcpy(), not taking into account the following define
> which is in arch/powerpc/include/asm/string.h (other arches do the same):
>
> #if defined(CONFIG_KASAN) && !defined(__SANITIZE_ADDRESS__)
> /*
> * For files that are not instrumented (e.g. mm/slub.c) we
> * should use not instrumented version of mem* functions.
> */
> #define memcpy(dst, src, len) __memcpy(dst, src, len)
> #define memmove(dst, src, len) __memmove(dst, src, len)
> #define memset(s, c, n) __memset(s, c, n)
> #endif
>
> void __init set_cur_cpu_spec(struct cpu_spec *s)
> {
> struct cpu_spec *t = &the_cpu_spec;
>
> t = PTRRELOC(t);
> *t = *s;
>
> *PTRRELOC(&cur_cpu_spec) = &the_cpu_spec;
> }
>
> 00000000 <set_cur_cpu_spec>:
> 0: 94 21 ff f0 stwu r1,-16(r1)
> 4: 7c 08 02 a6 mflr r0
> 8: bf c1 00 08 stmw r30,8(r1)
> c: 3f e0 00 00 lis r31,0
> e: R_PPC_ADDR16_HA .data..read_mostly
> 10: 3b ff 00 00 addi r31,r31,0
> 12: R_PPC_ADDR16_LO .data..read_mostly
> 14: 7c 7e 1b 78 mr r30,r3
> 18: 7f e3 fb 78 mr r3,r31
> 1c: 90 01 00 14 stw r0,20(r1)
> 20: 48 00 00 01 bl 20 <set_cur_cpu_spec+0x20>
> 20: R_PPC_REL24 add_reloc_offset
> 24: 7f c4 f3 78 mr r4,r30
> 28: 38 a0 00 58 li r5,88
> 2c: 48 00 00 01 bl 2c <set_cur_cpu_spec+0x2c>
> 2c: R_PPC_REL24 memcpy
> 30: 38 7f 00 58 addi r3,r31,88
> 34: 48 00 00 01 bl 34 <set_cur_cpu_spec+0x34>
> 34: R_PPC_REL24 add_reloc_offset
> 38: 93 e3 00 00 stw r31,0(r3)
> 3c: 80 01 00 14 lwz r0,20(r1)
> 40: bb c1 00 08 lmw r30,8(r1)
> 44: 7c 08 03 a6 mtlr r0
> 48: 38 21 00 10 addi r1,r1,16
> 4c: 4e 80 00 20 blr
>
>
> When replacing *t = *s by memcpy(t, s, sizeof(*t)), GCC replace it by
> __memcpy() as expected.
>
> >
> > Does using -ffreestanding and/or -fno-builtin-memcpy (-memset) help?
>
> No it doesn't and to be honest I can't see how it would. My
> understanding is that it could be even worse because it would mean
> adding calls to memcpy() also in all trivial places where GCC does the
> copy itself by default.
The idea was that with -ffreestanding compiler must not assume
presence of any runtime support library, so it must not emit any calls
that are not explicitly present in the source code. However, after
reading more docs, it seems that even with -ffreestanding gcc and
clang still assume presence of a runtime library that provides at
least memcpy, memmove, memset and memcmp. There does not seem to be a
way to prevent clang and gcc from doing it. So I guess this approach
is our only option:
Acked-by: Dmitry Vyukov <[email protected]>
Though, a comment may be useful so that a next person does not try to
revert it back.
> Do you see any alternative ?
>
> Christophe
>
> > If it helps, perhaps it makes sense to add these flags to
> > KASAN_SANITIZE := n files.
> >
> >
> >> *PTRRELOC(&cur_cpu_spec) = &the_cpu_spec;
> >> }
> >> @@ -2162,7 +2162,7 @@ static struct cpu_spec * __init setup_cpu_spec(unsigned long offset,
> >> old = *t;
> >>
> >> /* Copy everything, then do fixups */
> >> - *t = *s;
> >> + memcpy(t, s, sizeof(*t));
> >>
> >> /*
> >> * If we are overriding a previous value derived from the real
> >> diff --git a/arch/powerpc/kernel/setup_32.c b/arch/powerpc/kernel/setup_32.c
> >> index 947f904688b0..5e761eb16a6d 100644
> >> --- a/arch/powerpc/kernel/setup_32.c
> >> +++ b/arch/powerpc/kernel/setup_32.c
> >> @@ -73,10 +73,8 @@ notrace unsigned long __init early_init(unsigned long dt_ptr)
> >> {
> >> unsigned long offset = reloc_offset();
> >>
> >> - /* First zero the BSS -- use memset_io, some platforms don't have
> >> - * caches on yet */
> >> - memset_io((void __iomem *)PTRRELOC(&__bss_start), 0,
> >> - __bss_stop - __bss_start);
> >> + /* First zero the BSS */
> >> + memset(PTRRELOC(&__bss_start), 0, __bss_stop - __bss_start);
> >>
> >> /*
> >> * Identify the CPU type and fix up code sections
> >> --
> >> 2.13.3
> >>
On Tue, Jan 15, 2019 at 6:06 PM Andrey Ryabinin <[email protected]> wrote:
>
>
>
> On 1/15/19 2:14 PM, Dmitry Vyukov wrote:
> > On Tue, Jan 15, 2019 at 8:27 AM Christophe Leroy
> > <[email protected]> wrote:
> >> On 01/14/2019 09:34 AM, Dmitry Vyukov wrote:
> >>> On Sat, Jan 12, 2019 at 12:16 PM Christophe Leroy
> >>> <[email protected]> wrote:
> >>> >
> >>> > In kernel/cputable.c, explicitly use memcpy() in order
> >>> > to allow GCC to replace it with __memcpy() when KASAN is
> >>> > selected.
> >>> >
> >>> > Since commit 400c47d81ca38 ("powerpc32: memset: only use dcbz once cache is
> >>> > enabled"), memset() can be used before activation of the cache,
> >>> > so no need to use memset_io() for zeroing the BSS.
> >>> >
> >>> > Signed-off-by: Christophe Leroy <[email protected]>
> >>> > ---
> >>> > arch/powerpc/kernel/cputable.c | 4 ++--
> >>> > arch/powerpc/kernel/setup_32.c | 6 ++----
> >>> > 2 files changed, 4 insertions(+), 6 deletions(-)
> >>> >
> >>> > diff --git a/arch/powerpc/kernel/cputable.c
> >>> b/arch/powerpc/kernel/cputable.c
> >>> > index 1eab54bc6ee9..84814c8d1bcb 100644
> >>> > --- a/arch/powerpc/kernel/cputable.c
> >>> > +++ b/arch/powerpc/kernel/cputable.c
> >>> > @@ -2147,7 +2147,7 @@ void __init set_cur_cpu_spec(struct cpu_spec *s)
> >>> > struct cpu_spec *t = &the_cpu_spec;
> >>> >
> >>> > t = PTRRELOC(t);
> >>> > - *t = *s;
> >>> > + memcpy(t, s, sizeof(*t));
> >>>
> >>> Hi Christophe,
> >>>
> >>> I understand why you are doing this, but this looks a bit fragile and
> >>> non-scalable. This may not work with the next version of compiler,
> >>> just different than yours version of compiler, clang, etc.
> >>
> >> My felling would be that this change makes it more solid.
> >>
> >> My understanding is that when you do *t = *s, the compiler can use
> >> whatever way it wants to do the copy.
> >> When you do memcpy(), you ensure it will do it that way and not another
> >> way, don't you ?
> >
> > It makes this single line more deterministic wrt code-gen (though,
> > strictly saying compiler can turn memcpy back into inlines
> > instructions, it knows memcpy semantics anyway).
> > But the problem I meant is that the set of places that are subject to
> > this problem is not deterministic. So if we go with this solution,
> > after this change it's in the status "works on your machine" and we
> > either need to commit to not using struct copies and zeroing
> > throughout kernel code or potentially have a long tail of other
> > similar cases, and since they can be triggered by another compiler
> > version, we may need to backport these changes to previous releases
> > too. Whereas if we would go with compiler flags, it would prevent the
> > problem in all current and future places and with other past/future
> > versions of compilers.
> >
>
> The patch will work for any compiler. The point of this patch is to make
> memcpy() visible to the preprocessor which will replace it with __memcpy().
For this single line, yes. But it does not mean that KASAN will work.
> After preprocessor's work, compiler will see just __memcpy() call here.
Le 15/01/2019 à 18:10, Dmitry Vyukov a écrit :
> On Tue, Jan 15, 2019 at 6:06 PM Andrey Ryabinin <[email protected]> wrote:
>>
>>
>>
>> On 1/15/19 2:14 PM, Dmitry Vyukov wrote:
>>> On Tue, Jan 15, 2019 at 8:27 AM Christophe Leroy
>>> <[email protected]> wrote:
>>>> On 01/14/2019 09:34 AM, Dmitry Vyukov wrote:
>>>>> On Sat, Jan 12, 2019 at 12:16 PM Christophe Leroy
>>>>> <[email protected]> wrote:
>>>>> >
>>>>> > In kernel/cputable.c, explicitly use memcpy() in order
>>>>> > to allow GCC to replace it with __memcpy() when KASAN is
>>>>> > selected.
>>>>> >
>>>>> > Since commit 400c47d81ca38 ("powerpc32: memset: only use dcbz once cache is
>>>>> > enabled"), memset() can be used before activation of the cache,
>>>>> > so no need to use memset_io() for zeroing the BSS.
>>>>> >
>>>>> > Signed-off-by: Christophe Leroy <[email protected]>
>>>>> > ---
>>>>> > arch/powerpc/kernel/cputable.c | 4 ++--
>>>>> > arch/powerpc/kernel/setup_32.c | 6 ++----
>>>>> > 2 files changed, 4 insertions(+), 6 deletions(-)
>>>>> >
>>>>> > diff --git a/arch/powerpc/kernel/cputable.c
>>>>> b/arch/powerpc/kernel/cputable.c
>>>>> > index 1eab54bc6ee9..84814c8d1bcb 100644
>>>>> > --- a/arch/powerpc/kernel/cputable.c
>>>>> > +++ b/arch/powerpc/kernel/cputable.c
>>>>> > @@ -2147,7 +2147,7 @@ void __init set_cur_cpu_spec(struct cpu_spec *s)
>>>>> > struct cpu_spec *t = &the_cpu_spec;
>>>>> >
>>>>> > t = PTRRELOC(t);
>>>>> > - *t = *s;
>>>>> > + memcpy(t, s, sizeof(*t));
>>>>>
>>>>> Hi Christophe,
>>>>>
>>>>> I understand why you are doing this, but this looks a bit fragile and
>>>>> non-scalable. This may not work with the next version of compiler,
>>>>> just different than yours version of compiler, clang, etc.
>>>>
>>>> My felling would be that this change makes it more solid.
>>>>
>>>> My understanding is that when you do *t = *s, the compiler can use
>>>> whatever way it wants to do the copy.
>>>> When you do memcpy(), you ensure it will do it that way and not another
>>>> way, don't you ?
>>>
>>> It makes this single line more deterministic wrt code-gen (though,
>>> strictly saying compiler can turn memcpy back into inlines
>>> instructions, it knows memcpy semantics anyway).
>>> But the problem I meant is that the set of places that are subject to
>>> this problem is not deterministic. So if we go with this solution,
>>> after this change it's in the status "works on your machine" and we
>>> either need to commit to not using struct copies and zeroing
>>> throughout kernel code or potentially have a long tail of other
>>> similar cases, and since they can be triggered by another compiler
>>> version, we may need to backport these changes to previous releases
>>> too. Whereas if we would go with compiler flags, it would prevent the
>>> problem in all current and future places and with other past/future
>>> versions of compilers.
>>>
>>
>> The patch will work for any compiler. The point of this patch is to make
>> memcpy() visible to the preprocessor which will replace it with __memcpy().
>
> For this single line, yes. But it does not mean that KASAN will work.
>
>> After preprocessor's work, compiler will see just __memcpy() call here.
This problem can affect any arch I believe. Maybe the 'solution' would
be to run a generic script similar to
arch/powerpc/kernel/prom_init_check.sh on all objects compiled with
KASAN_SANITIZE_object.o := n don't include any reference to memcpy()
memset() or memmove() ?
Christophe
On 1/15/19 2:14 PM, Dmitry Vyukov wrote:
> On Tue, Jan 15, 2019 at 8:27 AM Christophe Leroy
> <[email protected]> wrote:
>> On 01/14/2019 09:34 AM, Dmitry Vyukov wrote:
>>> On Sat, Jan 12, 2019 at 12:16 PM Christophe Leroy
>>> <[email protected]> wrote:
>>> >
>>> > In kernel/cputable.c, explicitly use memcpy() in order
>>> > to allow GCC to replace it with __memcpy() when KASAN is
>>> > selected.
>>> >
>>> > Since commit 400c47d81ca38 ("powerpc32: memset: only use dcbz once cache is
>>> > enabled"), memset() can be used before activation of the cache,
>>> > so no need to use memset_io() for zeroing the BSS.
>>> >
>>> > Signed-off-by: Christophe Leroy <[email protected]>
>>> > ---
>>> > arch/powerpc/kernel/cputable.c | 4 ++--
>>> > arch/powerpc/kernel/setup_32.c | 6 ++----
>>> > 2 files changed, 4 insertions(+), 6 deletions(-)
>>> >
>>> > diff --git a/arch/powerpc/kernel/cputable.c
>>> b/arch/powerpc/kernel/cputable.c
>>> > index 1eab54bc6ee9..84814c8d1bcb 100644
>>> > --- a/arch/powerpc/kernel/cputable.c
>>> > +++ b/arch/powerpc/kernel/cputable.c
>>> > @@ -2147,7 +2147,7 @@ void __init set_cur_cpu_spec(struct cpu_spec *s)
>>> > struct cpu_spec *t = &the_cpu_spec;
>>> >
>>> > t = PTRRELOC(t);
>>> > - *t = *s;
>>> > + memcpy(t, s, sizeof(*t));
>>>
>>> Hi Christophe,
>>>
>>> I understand why you are doing this, but this looks a bit fragile and
>>> non-scalable. This may not work with the next version of compiler,
>>> just different than yours version of compiler, clang, etc.
>>
>> My felling would be that this change makes it more solid.
>>
>> My understanding is that when you do *t = *s, the compiler can use
>> whatever way it wants to do the copy.
>> When you do memcpy(), you ensure it will do it that way and not another
>> way, don't you ?
>
> It makes this single line more deterministic wrt code-gen (though,
> strictly saying compiler can turn memcpy back into inlines
> instructions, it knows memcpy semantics anyway).
> But the problem I meant is that the set of places that are subject to
> this problem is not deterministic. So if we go with this solution,
> after this change it's in the status "works on your machine" and we
> either need to commit to not using struct copies and zeroing
> throughout kernel code or potentially have a long tail of other
> similar cases, and since they can be triggered by another compiler
> version, we may need to backport these changes to previous releases
> too. Whereas if we would go with compiler flags, it would prevent the
> problem in all current and future places and with other past/future
> versions of compilers.
>
The patch will work for any compiler. The point of this patch is to make
memcpy() visible to the preprocessor which will replace it with __memcpy().
After preprocessor's work, compiler will see just __memcpy() call here.
On Tue, Jan 15, 2019 at 6:25 PM Christophe Leroy
<[email protected]> wrote:
>
> Le 15/01/2019 à 18:10, Dmitry Vyukov a écrit :
> > On Tue, Jan 15, 2019 at 6:06 PM Andrey Ryabinin <[email protected]> wrote:
> >>
> >> On 1/15/19 2:14 PM, Dmitry Vyukov wrote:
> >>> On Tue, Jan 15, 2019 at 8:27 AM Christophe Leroy
> >>> <[email protected]> wrote:
> >>>> On 01/14/2019 09:34 AM, Dmitry Vyukov wrote:
> >>>>> On Sat, Jan 12, 2019 at 12:16 PM Christophe Leroy
> >>>>> <[email protected]> wrote:
> >>>>> >
> >>>>> > In kernel/cputable.c, explicitly use memcpy() in order
> >>>>> > to allow GCC to replace it with __memcpy() when KASAN is
> >>>>> > selected.
> >>>>> >
> >>>>> > Since commit 400c47d81ca38 ("powerpc32: memset: only use dcbz once cache is
> >>>>> > enabled"), memset() can be used before activation of the cache,
> >>>>> > so no need to use memset_io() for zeroing the BSS.
> >>>>> >
> >>>>> > Signed-off-by: Christophe Leroy <[email protected]>
> >>>>> > ---
> >>>>> > arch/powerpc/kernel/cputable.c | 4 ++--
> >>>>> > arch/powerpc/kernel/setup_32.c | 6 ++----
> >>>>> > 2 files changed, 4 insertions(+), 6 deletions(-)
> >>>>> >
> >>>>> > diff --git a/arch/powerpc/kernel/cputable.c
> >>>>> b/arch/powerpc/kernel/cputable.c
> >>>>> > index 1eab54bc6ee9..84814c8d1bcb 100644
> >>>>> > --- a/arch/powerpc/kernel/cputable.c
> >>>>> > +++ b/arch/powerpc/kernel/cputable.c
> >>>>> > @@ -2147,7 +2147,7 @@ void __init set_cur_cpu_spec(struct cpu_spec *s)
> >>>>> > struct cpu_spec *t = &the_cpu_spec;
> >>>>> >
> >>>>> > t = PTRRELOC(t);
> >>>>> > - *t = *s;
> >>>>> > + memcpy(t, s, sizeof(*t));
> >>>>>
> >>>>> Hi Christophe,
> >>>>>
> >>>>> I understand why you are doing this, but this looks a bit fragile and
> >>>>> non-scalable. This may not work with the next version of compiler,
> >>>>> just different than yours version of compiler, clang, etc.
> >>>>
> >>>> My felling would be that this change makes it more solid.
> >>>>
> >>>> My understanding is that when you do *t = *s, the compiler can use
> >>>> whatever way it wants to do the copy.
> >>>> When you do memcpy(), you ensure it will do it that way and not another
> >>>> way, don't you ?
> >>>
> >>> It makes this single line more deterministic wrt code-gen (though,
> >>> strictly saying compiler can turn memcpy back into inlines
> >>> instructions, it knows memcpy semantics anyway).
> >>> But the problem I meant is that the set of places that are subject to
> >>> this problem is not deterministic. So if we go with this solution,
> >>> after this change it's in the status "works on your machine" and we
> >>> either need to commit to not using struct copies and zeroing
> >>> throughout kernel code or potentially have a long tail of other
> >>> similar cases, and since they can be triggered by another compiler
> >>> version, we may need to backport these changes to previous releases
> >>> too. Whereas if we would go with compiler flags, it would prevent the
> >>> problem in all current and future places and with other past/future
> >>> versions of compilers.
> >>>
> >>
> >> The patch will work for any compiler. The point of this patch is to make
> >> memcpy() visible to the preprocessor which will replace it with __memcpy().
> >
> > For this single line, yes. But it does not mean that KASAN will work.
> >
> >> After preprocessor's work, compiler will see just __memcpy() call here.
>
> This problem can affect any arch I believe. Maybe the 'solution' would
> be to run a generic script similar to
> arch/powerpc/kernel/prom_init_check.sh on all objects compiled with
> KASAN_SANITIZE_object.o := n don't include any reference to memcpy()
> memset() or memmove() ?
We do this when building user-space sanitizers runtime. There all code
always runs with sanitizer enabled, but at the same time must not be
instrumented. So we committed to changing all possible memcpy/memset
injection points and have a script that checks that we indeed have no
such calls at any paths. There problem is a bit simpler as we don't
have gazillion combinations of configs and the runtime is usually
self-hosted (as it is bundled with compiler), so we know what compiler
is used to build it. And that all is checked on CI.
I don't know how much work it is to do the same for kernel, though.
Adding -ffreestanding, if worked, looked like a cheap option to
achieve the same.
Another option is to insert checks into KASAN's memcpy/memset that at
least some early init has completed. If early init hasn't finished
yet, then they could skip all additional work besides just doing
memcpy/memset. We can't afford this for memory access instrumentation
for performance reasons, but it should be bearable for memcpy/memset.