The encoding of an x86 instruction can include a ModR/M and a SIB
(Scale-Index-Base) byte to describe the addressing mode of the
instruction.
objtool processes all addressing mode with a SIB base of 5 as having
%rbp as the base register. However, a SIB base of 5 means that the
effective address has either no base (if ModR/M mod is zero) or %rbp
as the base (if ModR/M mod is 1 or 2). This can cause objtool to confuse
an absolute address access with a stack operation.
For example, objtool will see the following instruction:
4c 8b 24 25 e0 ff ff mov 0xffffffffffffffe0,%r12
as a stack operation (i.e. similar to: mov -0x20(%rbp), %r12).
[Note that this kind of weird absolute address access is added by the
compiler when using KASAN.]
If this perceived stack operation happens to reference the location
where %r12 was pushed on the stack then the objtool validation will
think that %r12 is being restored and this can cause a stack state
mismatch.
This kind behavior was seen on xfs code, after a minor change (convert
kmem_alloc() to kmalloc()):
>> fs/xfs/xfs.o: warning: objtool: xfs_da_grow_inode_int+0x6c1: stack state mismatch: reg1[12]=-2-48 reg2[12]=-1+0
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Signed-off-by: Alexandre Chartre <[email protected]>
---
tools/objtool/arch/x86/decode.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 3a1d80a7878d3..18a9140173326 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -412,6 +412,14 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
if (!rex_w)
break;
+ /*
+ * If the SIB base is 5, and ModRM mod is 0 then there
+ * is no base. But SIB decoding will set sib_base to
+ * CFI_BP (register 5).
+ */
+ if (have_SIB() && sib_base == CFI_BP && modrm_mod == 0)
+ break;
+
if (rm_is_mem(CFI_BP)) {
/* mov disp(%rbp), reg */
--
2.39.3
On Thu, Mar 28, 2024 at 02:46:34PM +0100, Alexandre Chartre wrote:
> The encoding of an x86 instruction can include a ModR/M and a SIB
> (Scale-Index-Base) byte to describe the addressing mode of the
> instruction.
>
> objtool processes all addressing mode with a SIB base of 5 as having
> %rbp as the base register. However, a SIB base of 5 means that the
> effective address has either no base (if ModR/M mod is zero) or %rbp
> as the base (if ModR/M mod is 1 or 2). This can cause objtool to confuse
> an absolute address access with a stack operation.
>
> For example, objtool will see the following instruction:
>
> 4c 8b 24 25 e0 ff ff mov 0xffffffffffffffe0,%r12
>
> as a stack operation (i.e. similar to: mov -0x20(%rbp), %r12).
>
> [Note that this kind of weird absolute address access is added by the
> compiler when using KASAN.]
>
> If this perceived stack operation happens to reference the location
> where %r12 was pushed on the stack then the objtool validation will
> think that %r12 is being restored and this can cause a stack state
> mismatch.
>
> This kind behavior was seen on xfs code, after a minor change (convert
> kmem_alloc() to kmalloc()):
>
> >> fs/xfs/xfs.o: warning: objtool: xfs_da_grow_inode_int+0x6c1: stack state mismatch: reg1[12]=-2-48 reg2[12]=-1+0
>
> Reported-by: kernel test robot <[email protected]>
> Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
> Signed-off-by: Alexandre Chartre <[email protected]>
Nice, thanks for finding and debugging this.
Would it make sense to make the check more generic by putting it into
rm_is()?
--
Josh
On 3/29/24 02:39, Josh Poimboeuf wrote:
> On Thu, Mar 28, 2024 at 02:46:34PM +0100, Alexandre Chartre wrote:
>> The encoding of an x86 instruction can include a ModR/M and a SIB
>> (Scale-Index-Base) byte to describe the addressing mode of the
>> instruction.
>>
>> objtool processes all addressing mode with a SIB base of 5 as having
>> %rbp as the base register. However, a SIB base of 5 means that the
>> effective address has either no base (if ModR/M mod is zero) or %rbp
>> as the base (if ModR/M mod is 1 or 2). This can cause objtool to confuse
>> an absolute address access with a stack operation.
>>
>> For example, objtool will see the following instruction:
>>
>> 4c 8b 24 25 e0 ff ff mov 0xffffffffffffffe0,%r12
>>
>> as a stack operation (i.e. similar to: mov -0x20(%rbp), %r12).
>>
>> [Note that this kind of weird absolute address access is added by the
>> compiler when using KASAN.]
>>
>> If this perceived stack operation happens to reference the location
>> where %r12 was pushed on the stack then the objtool validation will
>> think that %r12 is being restored and this can cause a stack state
>> mismatch.
>>
>> This kind behavior was seen on xfs code, after a minor change (convert
>> kmem_alloc() to kmalloc()):
>>
>>>> fs/xfs/xfs.o: warning: objtool: xfs_da_grow_inode_int+0x6c1: stack state mismatch: reg1[12]=-2-48 reg2[12]=-1+0
>>
>> Reported-by: kernel test robot <[email protected]>
>> Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
>> Signed-off-by: Alexandre Chartre <[email protected]>
>
> Nice, thanks for finding and debugging this.
>
> Would it make sense to make the check more generic by putting it into
> rm_is()?
>
Yes. Making the change.
Thanks,
alex.