On Feb 26, 2015 1:55 AM, "Denys Vlasenko" <vda.li...@googlemail.com> wrote: > > On Wed, Feb 25, 2015 at 10:59 PM, Andy Lutomirski <l...@amacapital.net> wrote: > > On Wed, Feb 25, 2015 at 1:28 PM, Denys Vlasenko <dvlas...@redhat.com> wrote: > >> On 02/25/2015 09:10 PM, Andy Lutomirski wrote: > >> This part? > >> > >> .macro FORK_LIKE func > >> ENTRY(stub_\func) > >> CFI_STARTPROC > >> - popq %r11 /* save return address */ > >> - PARTIAL_FRAME 0 > >> - SAVE_REST > >> - pushq %r11 /* put it back on stack */ > >> + DEFAULT_FRAME 0, 8 /* offset 8: return address */ > >> + SAVE_EXTRA_REGS 8 > >> FIXUP_TOP_OF_STACK %r11, 8 > >> - DEFAULT_FRAME 0 8 /* offset 8: return address */ > >> call sys_\func > >> RESTORE_TOP_OF_STACK %r11, 8 > >> - ret $REST_SKIP /* pop extended registers */ > >> + ret > >> CFI_ENDPROC > >> END(stub_\func) > >> .endm > >> > >> FORK_LIKE clone > >> FORK_LIKE fork > >> FORK_LIKE vfork > >> > >> But the old code (SAVE_REST thing) was also saving registers here. > >> It had to jump through hoops (pop return address, SAVE_REST, > >> push return address) to do that. > >> After the patch, "SAVE_EXTRA_REGS 8" does the same, just without > >> pop/push pair. > >> > >> I just don't see what's wrong with it. Can you elaborate? > > > > SAVE_REST pushed the regs onto the stack, whereas SAVE_EXTRA_REGS just > > writes them in place. It's possible for this to be called when the > > regs have already been saved. > > If that would be the case - that is, if SAVE_REST was saving extra copy > of registers on stack, then FIXUP_TOP_OF_STACK %r11, 8 would be working > on wrong locations. The "8" there says "we have full pt_regs on stack, > plus extra 8 bytes (the return address)". Your conjecture would mean > that in fact there would be more bytes on stack, and FIXUP_TOP_OF_STACK > would corrupt iret stack. Evidently, since old code was not crashing, > this wasn't happening. SAVE_REST was really creating the "tail" of pt_regs
Ugh, you're right. The FIXUP_TOP_OF_STACK indeed looks duplicated, bit t that's less harmful and was already the case. --Andy . > > In addition to my previous tests, I ran my home machine with > patched kernel. Unfortunately, it works for me :( > > Will try on yet another machine. > > -- > vda -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
