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Date: Wed, 25 Oct 2017 13:25:02 +0200 From: Juergen Gross <jgross@...e.com> To: Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org Cc: linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Sasha Levin <alexander.levin@...izon.com>, live-patching@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>, Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, Mike Galbraith <efault@....de>, Chris Wright <chrisw@...s-sol.org>, Alok Kataria <akataria@...are.com>, Rusty Russell <rusty@...tcorp.com.au>, virtualization@...ts.linux-foundation.org, Boris Ostrovsky <boris.ostrovsky@...cle.com>, xen-devel@...ts.xenproject.org, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de> Subject: Re: [PATCH 10/13] x86/alternative: Support indirect call replacement 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@...hat.com> > --- > 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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