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Daniel Jacobowitz <drow@false.org> writes: > On Mon, Jul 17, 2006 at 09:29:46AM +0200, Mark Kettenis wrote: >> Something like what's done in the kernel (arch/x86_64/kernel/vsyscall.S). >> Hmm, I wonder why Daniel's box uses the trampoline from libc instead of >> the trampoline in the vsyscall page. > > Ah, now, this is a very interesting question. I'm glad you asked :-) > > __libc_sigaction (int sig, const struct sigaction *act, struct > sigaction *oact) > { > int result; > struct kernel_sigaction kact, koact; > > if (act) > { > kact.k_sa_handler = act->sa_handler; > memcpy (&kact.sa_mask, &act->sa_mask, sizeof (sigset_t)); > kact.sa_flags = act->sa_flags | SA_RESTORER; > > kact.sa_restorer = &restore_rt; > } > > That's how we end up at the trampoline: through use of SA_RESTORER. > I didn't respond to this earlier because I wanted to find some time to > check whether that was necessary. > > Andreas, looking at the i386 version, I guess that using SA_RESTORER > this way is not necessary. Simply a performance optimization because > the old trampolines (written to the stack) were so slow, or maybe > because they required an executable stack. i386 has > "if (GLRO(dl_sysinfo_dso) == NULL)" around it. Can x86_64 do the same > thing? i386 is the only platform doing it. I don't know the history of the change and whether this is the right thing to do. Is somebody willing to test this? > The existing unwind information would still be wrong, but on systems > with a vDSO it wouldn't matter any more. > >> Anyway, if with the current libc, the trampoline provided by the kernel is >> supposed to be used, then it's probably not worth bothering to add CFI >> to libc, and I'd just remove the CFI_STARTPROC and CFI_ENDPROC statements. > > Either way seems reasonable. Andreas -- Andreas Jaeger, aj@suse.de, http://www.suse.de/~aj/ SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany GPG fingerprint = 93A3 365E CE47 B889 DF7F FED1 389A 563C C272 A126
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