One-element arrays are deprecated, and we are replacing them with flexible
array members instead. So, replace one-element arrays with flexible-array
members in structures xfs_attr_leaf_name_local and
xfs_attr_leaf_name_remote.
The only binary differences reported after the changes are all like
these:
fs/xfs/libxfs/xfs_attr_leaf.o
_@@ -435,7 +435,7 @@
3b8: movzbl 0x2(%rbx),%eax
3bc: rol $0x8,%bp
3c0: movzwl %bp,%ebp
- 3c3: lea 0x2(%rax,%rbp,1),%ebx
+ 3c3: lea 0x3(%rax,%rbp,1),%ebx
3c7: call 3cc <xfs_attr_leaf_entsize+0x8c>
3c8: R_X86_64_PLT32 __tsan_func_exit-0x4
3cc: or $0x3,%ebx
_@@ -454,7 +454,7 @@
3ea: movzbl 0x8(%rbx),%ebx
3ee: call 3f3 <xfs_attr_leaf_entsize+0xb3>
3ef: R_X86_64_PLT32 __tsan_func_exit-0x4
- 3f3: add $0xa,%ebx
+ 3f3: add $0xb,%ebx
3f6: or $0x3,%ebx
3f9: add $0x1,%ebx
3fc: mov %ebx,%eax
similar changes in fs/xfs/scrub/attr.o and fs/xfs/xfs.o object files.
And the reason for this is because of the round_up() macro called in
functions xfs_attr_leaf_entsize_remote() and xfs_attr_leaf_entsize_local(),
which is compensanting for the one-byte reduction in size (due to the
flex-array transformation) of structures xfs_attr_leaf_name_remote and
xfs_attr_leaf_name_local. So, sizes remain the same before and after
changes.
This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].
Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/251
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
Signed-off-by: Gustavo A. R. Silva <[email protected]>
---
fs/xfs/libxfs/xfs_da_format.h | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/fs/xfs/libxfs/xfs_da_format.h b/fs/xfs/libxfs/xfs_da_format.h
index 25e2841084e1..e1e62ebb0c44 100644
--- a/fs/xfs/libxfs/xfs_da_format.h
+++ b/fs/xfs/libxfs/xfs_da_format.h
@@ -620,14 +620,14 @@ typedef struct xfs_attr_leaf_entry { /* sorted on key, not name */
typedef struct xfs_attr_leaf_name_local {
__be16 valuelen; /* number of bytes in value */
__u8 namelen; /* length of name bytes */
- __u8 nameval[1]; /* name/value bytes */
+ __u8 nameval[]; /* name/value bytes */
} xfs_attr_leaf_name_local_t;
typedef struct xfs_attr_leaf_name_remote {
__be32 valueblk; /* block number of value bytes */
__be32 valuelen; /* number of bytes in value */
__u8 namelen; /* length of name bytes */
- __u8 name[1]; /* name bytes */
+ __u8 name[]; /* name bytes */
} xfs_attr_leaf_name_remote_t;
typedef struct xfs_attr_leafblock {
@@ -747,13 +747,13 @@ xfs_attr3_leaf_name_local(xfs_attr_leafblock_t *leafp, int idx)
*/
static inline int xfs_attr_leaf_entsize_remote(int nlen)
{
- return round_up(sizeof(struct xfs_attr_leaf_name_remote) - 1 +
+ return round_up(sizeof(struct xfs_attr_leaf_name_remote) +
nlen, XFS_ATTR_LEAF_NAME_ALIGN);
}
static inline int xfs_attr_leaf_entsize_local(int nlen, int vlen)
{
- return round_up(sizeof(struct xfs_attr_leaf_name_local) - 1 +
+ return round_up(sizeof(struct xfs_attr_leaf_name_local) +
nlen + vlen, XFS_ATTR_LEAF_NAME_ALIGN);
}
--
2.34.1
On Thu, Feb 02, 2023 at 07:24:50PM -0600, Gustavo A. R. Silva wrote:
> One-element arrays are deprecated, and we are replacing them with flexible
> array members instead. So, replace one-element arrays with flexible-array
> members in structures xfs_attr_leaf_name_local and
> xfs_attr_leaf_name_remote.
>
> The only binary differences reported after the changes are all like
> these:
>
> fs/xfs/libxfs/xfs_attr_leaf.o
> _@@ -435,7 +435,7 @@
> 3b8: movzbl 0x2(%rbx),%eax
> 3bc: rol $0x8,%bp
> 3c0: movzwl %bp,%ebp
> - 3c3: lea 0x2(%rax,%rbp,1),%ebx
> + 3c3: lea 0x3(%rax,%rbp,1),%ebx
> 3c7: call 3cc <xfs_attr_leaf_entsize+0x8c>
> 3c8: R_X86_64_PLT32 __tsan_func_exit-0x4
> 3cc: or $0x3,%ebx
> _@@ -454,7 +454,7 @@
> 3ea: movzbl 0x8(%rbx),%ebx
> 3ee: call 3f3 <xfs_attr_leaf_entsize+0xb3>
> 3ef: R_X86_64_PLT32 __tsan_func_exit-0x4
> - 3f3: add $0xa,%ebx
> + 3f3: add $0xb,%ebx
> 3f6: or $0x3,%ebx
> 3f9: add $0x1,%ebx
> 3fc: mov %ebx,%eax
>
> similar changes in fs/xfs/scrub/attr.o and fs/xfs/xfs.o object files.
I usually turn off the sanitizers for the A/B build comparisons to make
it easier to read the results. It looks like it _grew_ in size here,
though?
> And the reason for this is because of the round_up() macro called in
> functions xfs_attr_leaf_entsize_remote() and xfs_attr_leaf_entsize_local(),
> which is compensanting for the one-byte reduction in size (due to the
> flex-array transformation) of structures xfs_attr_leaf_name_remote and
> xfs_attr_leaf_name_local. So, sizes remain the same before and after
> changes.
>
> This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> routines on memcpy() and help us make progress towards globally
> enabling -fstrict-flex-arrays=3 [1].
>
> Link: https://github.com/KSPP/linux/issues/79
> Link: https://github.com/KSPP/linux/issues/251
> Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
> Signed-off-by: Gustavo A. R. Silva <[email protected]>
If xfstests pass, this seems good to me. Thanks!
Reviewed-by: Kees Cook <[email protected]>
--
Kees Cook
On Thu, Feb 02, 2023 at 07:24:50PM -0600, Gustavo A. R. Silva wrote:
> One-element arrays are deprecated, and we are replacing them with flexible
> array members instead. So, replace one-element arrays with flexible-array
> members in structures xfs_attr_leaf_name_local and
> xfs_attr_leaf_name_remote.
>
> The only binary differences reported after the changes are all like
> these:
>
> fs/xfs/libxfs/xfs_attr_leaf.o
> _@@ -435,7 +435,7 @@
> 3b8: movzbl 0x2(%rbx),%eax
> 3bc: rol $0x8,%bp
> 3c0: movzwl %bp,%ebp
> - 3c3: lea 0x2(%rax,%rbp,1),%ebx
> + 3c3: lea 0x3(%rax,%rbp,1),%ebx
> 3c7: call 3cc <xfs_attr_leaf_entsize+0x8c>
> 3c8: R_X86_64_PLT32 __tsan_func_exit-0x4
> 3cc: or $0x3,%ebx
> _@@ -454,7 +454,7 @@
> 3ea: movzbl 0x8(%rbx),%ebx
> 3ee: call 3f3 <xfs_attr_leaf_entsize+0xb3>
> 3ef: R_X86_64_PLT32 __tsan_func_exit-0x4
> - 3f3: add $0xa,%ebx
> + 3f3: add $0xb,%ebx
> 3f6: or $0x3,%ebx
> 3f9: add $0x1,%ebx
> 3fc: mov %ebx,%eax
>
> similar changes in fs/xfs/scrub/attr.o and fs/xfs/xfs.o object files.
>
> And the reason for this is because of the round_up() macro called in
> functions xfs_attr_leaf_entsize_remote() and xfs_attr_leaf_entsize_local(),
> which is compensanting for the one-byte reduction in size (due to the
> flex-array transformation) of structures xfs_attr_leaf_name_remote and
> xfs_attr_leaf_name_local. So, sizes remain the same before and after
> changes.
>
> This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> routines on memcpy() and help us make progress towards globally
> enabling -fstrict-flex-arrays=3 [1].
>
> Link: https://github.com/KSPP/linux/issues/79
> Link: https://github.com/KSPP/linux/issues/251
> Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
> Signed-off-by: Gustavo A. R. Silva <[email protected]>
> ---
> fs/xfs/libxfs/xfs_da_format.h | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_da_format.h b/fs/xfs/libxfs/xfs_da_format.h
> index 25e2841084e1..e1e62ebb0c44 100644
> --- a/fs/xfs/libxfs/xfs_da_format.h
> +++ b/fs/xfs/libxfs/xfs_da_format.h
> @@ -620,14 +620,14 @@ typedef struct xfs_attr_leaf_entry { /* sorted on key, not name */
> typedef struct xfs_attr_leaf_name_local {
> __be16 valuelen; /* number of bytes in value */
> __u8 namelen; /* length of name bytes */
> - __u8 nameval[1]; /* name/value bytes */
> + __u8 nameval[]; /* name/value bytes */
> } xfs_attr_leaf_name_local_t;
>
> typedef struct xfs_attr_leaf_name_remote {
> __be32 valueblk; /* block number of value bytes */
> __be32 valuelen; /* number of bytes in value */
> __u8 namelen; /* length of name bytes */
> - __u8 name[1]; /* name bytes */
> + __u8 name[]; /* name bytes */
Does the large comment about m68k problems in xfs_ondisk.h need updating
here?
--D
> } xfs_attr_leaf_name_remote_t;
>
> typedef struct xfs_attr_leafblock {
> @@ -747,13 +747,13 @@ xfs_attr3_leaf_name_local(xfs_attr_leafblock_t *leafp, int idx)
> */
> static inline int xfs_attr_leaf_entsize_remote(int nlen)
> {
> - return round_up(sizeof(struct xfs_attr_leaf_name_remote) - 1 +
> + return round_up(sizeof(struct xfs_attr_leaf_name_remote) +
> nlen, XFS_ATTR_LEAF_NAME_ALIGN);
> }
>
> static inline int xfs_attr_leaf_entsize_local(int nlen, int vlen)
> {
> - return round_up(sizeof(struct xfs_attr_leaf_name_local) - 1 +
> + return round_up(sizeof(struct xfs_attr_leaf_name_local) +
> nlen + vlen, XFS_ATTR_LEAF_NAME_ALIGN);
> }
>
> --
> 2.34.1
>
On Thu, Feb 02, 2023 at 07:24:50PM -0600, Gustavo A. R. Silva wrote:
> One-element arrays are deprecated, and we are replacing them with flexible
> array members instead. So, replace one-element arrays with flexible-array
> members in structures xfs_attr_leaf_name_local and
> xfs_attr_leaf_name_remote.
>
> The only binary differences reported after the changes are all like
> these:
>
> fs/xfs/libxfs/xfs_attr_leaf.o
> _@@ -435,7 +435,7 @@
> 3b8: movzbl 0x2(%rbx),%eax
> 3bc: rol $0x8,%bp
> 3c0: movzwl %bp,%ebp
> - 3c3: lea 0x2(%rax,%rbp,1),%ebx
> + 3c3: lea 0x3(%rax,%rbp,1),%ebx
> 3c7: call 3cc <xfs_attr_leaf_entsize+0x8c>
> 3c8: R_X86_64_PLT32 __tsan_func_exit-0x4
> 3cc: or $0x3,%ebx
> _@@ -454,7 +454,7 @@
> 3ea: movzbl 0x8(%rbx),%ebx
> 3ee: call 3f3 <xfs_attr_leaf_entsize+0xb3>
> 3ef: R_X86_64_PLT32 __tsan_func_exit-0x4
> - 3f3: add $0xa,%ebx
> + 3f3: add $0xb,%ebx
> 3f6: or $0x3,%ebx
> 3f9: add $0x1,%ebx
> 3fc: mov %ebx,%eax
>
> similar changes in fs/xfs/scrub/attr.o and fs/xfs/xfs.o object files.
That seems like a red flag to me - an off-by-one change in the
compiled code that calculates of the on-disk size of a structure as
a result of an in-memory structure change just smells like a bug.
How did you test this change?
> And the reason for this is because of the round_up() macro called in
> functions xfs_attr_leaf_entsize_remote() and xfs_attr_leaf_entsize_local(),
> which is compensanting for the one-byte reduction in size (due to the
> flex-array transformation) of structures xfs_attr_leaf_name_remote and
> xfs_attr_leaf_name_local. So, sizes remain the same before and after
> changes.
I'm not sure that is true. Before this change:
sizeof(xfs_attr_leaf_name_local_t) = 4
sizeof(xfs_attr_leaf_name_remote_t) = 12
After this change:
sizeof(xfs_attr_leaf_name_local_t) = 4
sizeof(xfs_attr_leaf_name_remote_t) = 12
i.e. no change because the structures aren't defined as packed
structures. Hence the compiler pads them to out to 4 byte alignment
naturally regardless of the flex array definition. pahole on x86-64
also confirms that the (padded) size of the structure is not
changed.
However, the on-disk structure it is being used to decode is packed,
and we're only using pointer arithmetic to pull the location of the
name/value pairs out of the buffer to copy them - it's the structure
size calculations that actually define the size of the structures
for a given name length, not the sizeof() value or the flex array
definitions...
> This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> routines on memcpy() and help us make progress towards globally
> enabling -fstrict-flex-arrays=3 [1].
>
> Link: https://github.com/KSPP/linux/issues/79
> Link: https://github.com/KSPP/linux/issues/251
> Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
> Signed-off-by: Gustavo A. R. Silva <[email protected]>
> ---
> fs/xfs/libxfs/xfs_da_format.h | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_da_format.h b/fs/xfs/libxfs/xfs_da_format.h
> index 25e2841084e1..e1e62ebb0c44 100644
> --- a/fs/xfs/libxfs/xfs_da_format.h
> +++ b/fs/xfs/libxfs/xfs_da_format.h
> @@ -620,14 +620,14 @@ typedef struct xfs_attr_leaf_entry { /* sorted on key, not name */
> typedef struct xfs_attr_leaf_name_local {
> __be16 valuelen; /* number of bytes in value */
> __u8 namelen; /* length of name bytes */
> - __u8 nameval[1]; /* name/value bytes */
> + __u8 nameval[]; /* name/value bytes */
> } xfs_attr_leaf_name_local_t;
>
> typedef struct xfs_attr_leaf_name_remote {
> __be32 valueblk; /* block number of value bytes */
> __be32 valuelen; /* number of bytes in value */
> __u8 namelen; /* length of name bytes */
> - __u8 name[1]; /* name bytes */
> + __u8 name[]; /* name bytes */
> } xfs_attr_leaf_name_remote_t;
>
> typedef struct xfs_attr_leafblock {
> @@ -747,13 +747,13 @@ xfs_attr3_leaf_name_local(xfs_attr_leafblock_t *leafp, int idx)
> */
> static inline int xfs_attr_leaf_entsize_remote(int nlen)
> {
> - return round_up(sizeof(struct xfs_attr_leaf_name_remote) - 1 +
> + return round_up(sizeof(struct xfs_attr_leaf_name_remote) +
> nlen, XFS_ATTR_LEAF_NAME_ALIGN);
> }
To be honest, the actual padding and alignment calculations are
kinda whacky because that's the way they were defined back in 1995.
And, well, once set in the on-disk format, it can't easily be
changed. FYI, here's the original definition from 1995:
#define XFS_ATTR_LEAF_ENTSIZE_REMOTE(nlen) /* space for remote struct */ \
(((sizeof(xfs_attr_leaf_name_remote_t)-1 + (nlen)) +3)&~0x3)
So apart using round_up and defines instead of magic numbers, the
current calculation is unchanged from the original definition.
AFAICT, the modification you are proposing above breaks this because the
sizeof(xfs_attr_leaf_name_remote) result has not changed with the
change of the structure definition.
e.g. if namelen = 17, before we had:
size = round_up(12 - 1 + 17, 4)
= round_up(28, 4)
= 28
Which is correct because the on-disk format is packed:
0 4 89 12 20 26 28
+---+---++--+-------+-----+-+-----....
|---------------| 17 bytes of name.
|-| 2 bytes of padding
|-----.... Next attr record.
We end up with 2 bytes of padded between the end of the name and the
start of the next attribute record in the block.
But after this patch, now we calculate the size as:
size = round_up(12 + 17, 4)
= round_up(29, 4)
= 32
Which is a different result, and would result in incorrect parsing
of the attribute records in the buffer. Hence I don't think it is
valid to be changing the entsize calculations like this if sizeof()
is not changing results.
Which comes back to my original question: how did you test this?
Cheers,
Dave.
--
Dave Chinner
[email protected]
On 2/5/23 16:51, Dave Chinner wrote:
> On Thu, Feb 02, 2023 at 07:24:50PM -0600, Gustavo A. R. Silva wrote:
>> One-element arrays are deprecated, and we are replacing them with flexible
>> array members instead. So, replace one-element arrays with flexible-array
>> members in structures xfs_attr_leaf_name_local and
>> xfs_attr_leaf_name_remote.
>>
>> The only binary differences reported after the changes are all like
>> these:
>>
>> fs/xfs/libxfs/xfs_attr_leaf.o
>> _@@ -435,7 +435,7 @@
>> 3b8: movzbl 0x2(%rbx),%eax
>> 3bc: rol $0x8,%bp
>> 3c0: movzwl %bp,%ebp
>> - 3c3: lea 0x2(%rax,%rbp,1),%ebx
>> + 3c3: lea 0x3(%rax,%rbp,1),%ebx
>> 3c7: call 3cc <xfs_attr_leaf_entsize+0x8c>
>> 3c8: R_X86_64_PLT32 __tsan_func_exit-0x4
>> 3cc: or $0x3,%ebx
>> _@@ -454,7 +454,7 @@
>> 3ea: movzbl 0x8(%rbx),%ebx
>> 3ee: call 3f3 <xfs_attr_leaf_entsize+0xb3>
>> 3ef: R_X86_64_PLT32 __tsan_func_exit-0x4
>> - 3f3: add $0xa,%ebx
>> + 3f3: add $0xb,%ebx
>> 3f6: or $0x3,%ebx
>> 3f9: add $0x1,%ebx
>> 3fc: mov %ebx,%eax
>>
>> similar changes in fs/xfs/scrub/attr.o and fs/xfs/xfs.o object files.
>
> That seems like a red flag to me - an off-by-one change in the
> compiled code that calculates of the on-disk size of a structure as
> a result of an in-memory structure change just smells like a bug.
Ughh..
You're right. I somehow got confused between the moment I first
build-tested this in my build machine and after a final last-minute
review I did on the machine from which I ultimately send the patches
out.
More comments below...
>
> How did you test this change?
>
>> And the reason for this is because of the round_up() macro called in
>> functions xfs_attr_leaf_entsize_remote() and xfs_attr_leaf_entsize_local(),
>> which is compensanting for the one-byte reduction in size (due to the
>> flex-array transformation) of structures xfs_attr_leaf_name_remote and
>> xfs_attr_leaf_name_local. So, sizes remain the same before and after
>> changes.
>
> I'm not sure that is true. Before this change:
Yeah; this in fact was a final last-minute review I did before sending out
the patch, and it was when I noticed the round_up() macro was doing something
quite idiomatic when it comes to calculating the sizes of structures containing
one-element arrays. People usually subtract the sizeof(type-of-one-element)
from the sizeof(struct-with-one-element-array) when they perform other
calculations. And in this case as the sizeof(type-of-one-element) is one byte,
at the moment I thought that subtraction was because of that, and then when I
build-tested that final change, I totally forgot about the padding (I had
actually noticed it when I modified the structure definitions :/) and now I
see I got all confused.
>
> sizeof(xfs_attr_leaf_name_local_t) = 4
> sizeof(xfs_attr_leaf_name_remote_t) = 12
>
> After this change:
>
> sizeof(xfs_attr_leaf_name_local_t) = 4
> sizeof(xfs_attr_leaf_name_remote_t) = 12
Yes; in fact I noticed that. :/
>
> i.e. no change because the structures aren't defined as packed
> structures. Hence the compiler pads them to out to 4 byte alignment
> naturally regardless of the flex array definition. pahole on x86-64
> also confirms that the (padded) size of the structure is not
> changed.
Yep; I actually was going to include the pahole output for both structures
in the changelog text, but I decided not to do it at the last minute as
I didn't see it necessary because, as you pointed out, the sizes before
and after the flex-array transformations are the same.
>
> However, the on-disk structure it is being used to decode is packed,
> and we're only using pointer arithmetic to pull the location of the
> name/value pairs out of the buffer to copy them - it's the structure
> size calculations that actually define the size of the structures
> for a given name length, not the sizeof() value or the flex array
> definitions...
>
>> This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
>> routines on memcpy() and help us make progress towards globally
>> enabling -fstrict-flex-arrays=3 [1].
>>
>> Link: https://github.com/KSPP/linux/issues/79
>> Link: https://github.com/KSPP/linux/issues/251
>> Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
>> Signed-off-by: Gustavo A. R. Silva <[email protected]>
>> ---
>> fs/xfs/libxfs/xfs_da_format.h | 8 ++++----
>> 1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/xfs/libxfs/xfs_da_format.h b/fs/xfs/libxfs/xfs_da_format.h
>> index 25e2841084e1..e1e62ebb0c44 100644
>> --- a/fs/xfs/libxfs/xfs_da_format.h
>> +++ b/fs/xfs/libxfs/xfs_da_format.h
>> @@ -620,14 +620,14 @@ typedef struct xfs_attr_leaf_entry { /* sorted on key, not name */
>> typedef struct xfs_attr_leaf_name_local {
>> __be16 valuelen; /* number of bytes in value */
>> __u8 namelen; /* length of name bytes */
>> - __u8 nameval[1]; /* name/value bytes */
>> + __u8 nameval[]; /* name/value bytes */
>> } xfs_attr_leaf_name_local_t;
>>
>> typedef struct xfs_attr_leaf_name_remote {
>> __be32 valueblk; /* block number of value bytes */
>> __be32 valuelen; /* number of bytes in value */
>> __u8 namelen; /* length of name bytes */
>> - __u8 name[1]; /* name bytes */
>> + __u8 name[]; /* name bytes */
>> } xfs_attr_leaf_name_remote_t;
>>
>> typedef struct xfs_attr_leafblock {
>> @@ -747,13 +747,13 @@ xfs_attr3_leaf_name_local(xfs_attr_leafblock_t *leafp, int idx)
>> */
>> static inline int xfs_attr_leaf_entsize_remote(int nlen)
>> {
>> - return round_up(sizeof(struct xfs_attr_leaf_name_remote) - 1 +
>> + return round_up(sizeof(struct xfs_attr_leaf_name_remote) +
>> nlen, XFS_ATTR_LEAF_NAME_ALIGN);
>> }
>
> To be honest, the actual padding and alignment calculations are
> kinda whacky because that's the way they were defined back in 1995.
> And, well, once set in the on-disk format, it can't easily be
> changed. FYI, here's the original definition from 1995:
>
> #define XFS_ATTR_LEAF_ENTSIZE_REMOTE(nlen) /* space for remote struct */ \
> (((sizeof(xfs_attr_leaf_name_remote_t)-1 + (nlen)) +3)&~0x3)
>
> So apart using round_up and defines instead of magic numbers, the
> current calculation is unchanged from the original definition.
>
> AFAICT, the modification you are proposing above breaks this because the
> sizeof(xfs_attr_leaf_name_remote) result has not changed with the
> change of the structure definition.
>
> e.g. if namelen = 17, before we had:
>
> size = round_up(12 - 1 + 17, 4)
> = round_up(28, 4)
> = 28
>
> Which is correct because the on-disk format is packed:
>
> 0 4 89 12 20 26 28
> +---+---++--+-------+-----+-+-----....
> |---------------| 17 bytes of name.
> |-| 2 bytes of padding
> |-----.... Next attr record.
>
> We end up with 2 bytes of padded between the end of the name and the
> start of the next attribute record in the block.
>
> But after this patch, now we calculate the size as:
>
> size = round_up(12 + 17, 4)
> = round_up(29, 4)
> = 32
>
> Which is a different result, and would result in incorrect parsing
> of the attribute records in the buffer. Hence I don't think it is
> valid to be changing the entsize calculations like this if sizeof()
> is not changing results.
Yep; you're right.
>
> Which comes back to my original question: how did you test this?
I compared the generated object files in fs/xfs/, fs/xfs/scrub/ and
fs/xfs/libxfs/ before and after the changes with something like
these[1]:
ARGS=--disassemble --demangle --reloc --no-show-raw-insn --section=.text
for i in $(cd $OUT/xfs/before && echo *.o); do echo $i; diff -u <(objdump $ARGS $OUT/xfs/before/$i | sed "0,/^Disassembly/d") <(objdump $ARGS $OUT/xfs/after/$i
| sed "0,/^Disassembly/d"); done
where of course the generated object files before the changes are
located in OUT/xfs/before/ and the ones after changes in $OUT/xfs/after/
I just double-checked and, indeed, the changes I mentioned in the
changelog text only show up when I modify the entsize functions.
So, because of the padding, the flex-array transformations don't
actually affect the sizes of the involved structures. So, it seems
that change is enough and is the correct one.
I really appreciate your comments and feedback, Dave. And I'm sorry
for the confusion.
Thank you!
--
Gustavo
[1] https://outflux.net/blog/archives/2022/06/24/finding-binary-differences/
On 2/3/23 11:53, Kees Cook wrote:
> On Thu, Feb 02, 2023 at 07:24:50PM -0600, Gustavo A. R. Silva wrote:
>> One-element arrays are deprecated, and we are replacing them with flexible
>> array members instead. So, replace one-element arrays with flexible-array
>> members in structures xfs_attr_leaf_name_local and
>> xfs_attr_leaf_name_remote.
>>
>> The only binary differences reported after the changes are all like
>> these:
>>
>> fs/xfs/libxfs/xfs_attr_leaf.o
>> _@@ -435,7 +435,7 @@
>> 3b8: movzbl 0x2(%rbx),%eax
>> 3bc: rol $0x8,%bp
>> 3c0: movzwl %bp,%ebp
>> - 3c3: lea 0x2(%rax,%rbp,1),%ebx
>> + 3c3: lea 0x3(%rax,%rbp,1),%ebx
>> 3c7: call 3cc <xfs_attr_leaf_entsize+0x8c>
>> 3c8: R_X86_64_PLT32 __tsan_func_exit-0x4
>> 3cc: or $0x3,%ebx
>> _@@ -454,7 +454,7 @@
>> 3ea: movzbl 0x8(%rbx),%ebx
>> 3ee: call 3f3 <xfs_attr_leaf_entsize+0xb3>
>> 3ef: R_X86_64_PLT32 __tsan_func_exit-0x4
>> - 3f3: add $0xa,%ebx
>> + 3f3: add $0xb,%ebx
>> 3f6: or $0x3,%ebx
>> 3f9: add $0x1,%ebx
>> 3fc: mov %ebx,%eax
>>
>> similar changes in fs/xfs/scrub/attr.o and fs/xfs/xfs.o object files.
>
> I usually turn off the sanitizers for the A/B build comparisons to make
Oh yes! that's a good point. I'll see that they are turned off next time. :)
> it easier to read the results. It looks like it _grew_ in size here,
> though?
Yep; I'm sorry I got it wrong. :/ I had it right in the beginning, then after
reading the code once again just before sending out a version of this patch
with only the flex-array transformations, I noticed the entsize functions and
the "sizeof(struct-with-one-element-array) - 1" and I forgot about the padding,
removed the "- 1" and got a bit confused with my build-tests.
I'll send v2 with my original changes... the flex-array transformations, only.
--
Gustavo