Messages in this thread | | | Date | Wed, 8 Aug 2012 16:53:45 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/5] x86/uprobes: implement x86 specific arch_uprobe_*_step |
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On 08/08, Sebastian Andrzej Siewior wrote: > > On 08/08/2012 02:57 PM, Oleg Nesterov wrote: >>> +static int insn_changes_flags(struct arch_uprobe *auprobe) >>> +{ >>> + /* popf reads flags from stack */ >>> + if (auprobe->insn[0] == 0x9d) >>> + return 1; >> >> Ah, somehow I didn't think about this before. >> >> ->insn[0] doesn't look right, we should skip the prefixes. > > Why? I tried 'lock popf' and I got invalid instruction. The same for > 'rep popf'.
int main(void) { asm volatile ("pushf; rep; popf");
return 0; }
objdump:
00000000040047c <main>: 40047c: 55 push %rbp 40047d: 48 89 e5 mov %rsp,%rbp 400480: 9c pushfq 400481: f3 9d repz popfq 400483: b8 00 00 00 00 mov $0x0,%eax 400488: c9 leaveq 400489: c3 retq
OK, probably nobody should do this (although the kernel should not assume this imho), but
asm volatile ("pushfw; popfw");
doesn't look bad and the code is
000000000040047c <main>: 40047c: 55 push %rbp 40047d: 48 89 e5 mov %rsp,%rbp 400480: 66 9c pushfw 400482: 66 9d popfw 400484: b8 00 00 00 00 mov $0x0,%eax 400489: c9 leaveq 40048a: c3 retq
And in any case it would be better to re-use auprobe->fixups.
Oleg.
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