Re: x86_64 frame pointer via thread context

[Petr Vandrovec <vandrove@vc.cvut.cz>]

Dave Jiang wrote:
> Petr Vandrovec wrote:
> > #0  tb_sig_handler (sig=33, info=0x407ff2f0, ucontext=0x407ff1c0) at 
> > ttest1.c:26
> > #1  <signal handler called>
> > #2  0x00002aaaaad81335 in nanosleep () from /lib/libc.so.6
> > #3  0x00002aaaaad811a5 in sleep () from /lib/libc.so.6
> > #4  0x0000000000400871 in test_thread1 (arg=0x0) at ttest1.c:40
> > #5  0x00002aaaaabc6b55 in start_thread () from /lib/libpthread.so.0
> > #6  0x00002aaaaada87f0 in clone () from /lib/libc.so.6
>
>
> Ooops, you are right. I forgot about those ones in the threads. Yes you 
> are right. Once the sleep goes away rBP displays the correct values. Is 
> this issue due to glibc or because of the toolchain? I do not have this 
> issues on 32bit x86.... I would assume that the reason it works on 
> Mandrake is due to the toolchain they use versus other distros? The 
> toolchain determines which registers to use and the 
> -fno-omit-frame-pointer did not prevent some of them from clobbering rbp?

You are building only your application with -fno-omit-frame-pointer,
libraries you are using are just used as is.  On 32bit x86 it works
as -O2 on x86 does not imply -fomit-frame-pointer, as frame pointer
is required (well, was) for debugging.  On x86-64 frame pointer is
not needed for debugging as it was always using DWARF debug info,
and so -O2 on x86-64 implies -fomit-frame-pointer.  Due to this most
of libraries you'll find on your 64bit system are built without frame
pointer.  Mandrake either explicitly asks for -fno-omit-frame-pointer,
or maybe their glibc is just sufficiently different that their sleep()
does not need %rbp for sleep().

Loading their glibc to the debugger and inspecting sleep & nanosleep
will reveal whether %rbp is just unchanged by these (and so it works
due to luck), or whether their sleep uses pushq %rbp; movq %rsp,%rbp
- in which case they built glibc with frame pointers for no apparent
reason.

You must use debugging informations to get stacktrace on x86-64.
Blindly following %rbp does not work on this architecture (and lot
of others).
							Petr

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