On 04/10/17 17:58, Josh Poimboeuf wrote:
> Add alternative patching support for replacing an instruction with an
> indirect call. This will be needed for the paravirt alternatives.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---
> arch/x86/kernel/alternative.c | 22 +++++++++++++++-------
> 1 file changed, 15 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 3344d3382e91..81c577c7deba 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -410,20 +410,28 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
> insnbuf_sz = a->replacementlen;
>
> /*
> - * 0xe8 is a relative jump; fix the offset.
> - *
> - * Instruction length is checked before the opcode to avoid
> - * accessing uninitialized bytes for zero-length replacements.
> + * Fix the address offsets for call and jump instructions which
> + * use PC-relative addressing.
> */
> if (a->replacementlen == 5 && *insnbuf == 0xe8) {
> + /* direct call */
> *(s32 *)(insnbuf + 1) += replacement - instr;
> - DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
> + DPRINTK("Fix direct CALL offset: 0x%x, CALL 0x%lx",
> *(s32 *)(insnbuf + 1),
> (unsigned long)instr + *(s32 *)(insnbuf + 1) + 5);
> - }
>
> - if (a->replacementlen && is_jmp(replacement[0]))
> + } else if (a->replacementlen == 6 && *insnbuf == 0xff &&
> + *(insnbuf+1) == 0x15) {
> + /* indirect call */
> + *(s32 *)(insnbuf + 2) += replacement - instr;
> + DPRINTK("Fix indirect CALL offset: 0x%x, CALL *0x%lx",
> + *(s32 *)(insnbuf + 2),
> + (unsigned long)instr + *(s32 *)(insnbuf + 2) + 6);
> +
> + } else if (a->replacementlen && is_jmp(replacement[0])) {
Is this correct? Without your patch this was:
if (*insnbuf == 0xe8) ...
if (is_jmp(replacement[0])) ...
Now you have:
if (*insnbuf == 0xe8) ...
else if (*insnbuf == 0xff15) ...
else if (is_jmp(replacement[0])) ...
So only one or none of the three variants will be executed. In the past
it could be none, one or both.
Juergen
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