Messages in this thread | | | Date | Thu, 28 Mar 2024 18:39:32 -0700 | From | Josh Poimboeuf <> | Subject | Re: [PATCH] objtool/x86: objtool can confuse memory and stack access |
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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 <lkp@intel.com> > Closes: https://lore.kernel.org/oe-kbuild-all/202402220435.MGN0EV6l-lkp@intel.com/ > Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Nice, thanks for finding and debugging this.
Would it make sense to make the check more generic by putting it into rm_is()?
-- Josh
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