internal backtrace support - libunwind or backtrace_symbols?

[Andrew Bartlett]

Michael,

You mentioned that you didn't get a backtrace until buidling debian
packages with libunwind.  Can you double-check?  The reason I ask is
that when I disabled libunwind with this MR, I get the
backtrace_symbols one, and I need to know if this isn't working in the
'real world'.

https://gitlab.com/samba-team/samba/-/merge_requests/2534

To the list more generally:

I'm trying to merge the above MR from Fco Javier Felix and have been
doing some testing.

We currently prefer the extra library libunwind for backtraces, but the
backtrace on modern linux (Ubuntu 20.04) is actually better from
backtrace_symbols():

backtrace_symbols:
 #0 bin/shared/libsamba-util.so.0(log_stack_trace+0x32)
[0x7f038a9ba096]
 #1 bin/shared/libsamba-util.so.0(smb_panic_log+0x1b5) [0x7f038a9ba013]
 #2 bin/shared/libsamba-util.so.0(smb_panic+0x1c) [0x7f038a9ba032]
 #3 bin/shared/libsamba-util.so.0(+0x10b36) [0x7f038a9b9b36]
 #4 bin/shared/libsamba-util.so.0(+0x10b4f) [0x7f038a9b9b4f]
 #5 /lib/x86_64-linux-gnu/libpthread.so.0(+0x14420) [0x7f038a51f420]
 #6 /lib/x86_64-linux-gnu/libc.so.6(epoll_wait+0x1a) [0x7f038a43842a]
 #7 bin/shared/private/libtevent-samba4.so(+0x12e96) [0x7f038ab0fe96]
 #8 bin/shared/private/libtevent-samba4.so(+0x13864) [0x7f038ab10864]
 #9 bin/shared/private/libtevent-samba4.so(+0xff1d) [0x7f038ab0cf1d]
 #10 bin/shared/private/libtevent-samba4.so(_tevent_loop_once+0x126)
[0x7f038ab038c8]
 #11 bin/shared/private/libtevent-
samba4.so(tevent_common_loop_wait+0x29) [0x7f038ab03c32]
 #12 bin/shared/private/libtevent-samba4.so(+0xffc3) [0x7f038ab0cfc3]
 #13 bin/shared/private/libtevent-samba4.so(_tevent_loop_wait+0x2f)
[0x7f038ab03cd9]
 #14 samba: root process(+0x784e) [0x5631fbd6a84e]
 #15 samba: root process(main+0x76) [0x5631fbd6a911]
 #16 /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf3)
[0x7f038a33d083]
 #17 samba: root process(_start+0x2e) [0x5631fbd67f4e]

libunwind:
 #0 log_stack_trace + 0x3b [ip=0x7f272f0de0ff] [sp=0x7ffc2818af20]
 #1 smb_panic_log + 0x1b5 [ip=0x7f272f0de073] [sp=0x7ffc2818b830]
 #2 smb_panic + 0x1c [ip=0x7f272f0de092] [sp=0x7ffc2818b850]
 #3 fault_report + 0x91 [ip=0x7f272f0ddb96] [sp=0x7ffc2818b870]
 #4 sig_fault + 0x19 [ip=0x7f272f0ddbaf] [sp=0x7ffc2818b920]
 #5 funlockfile + 0x60 [ip=0x7f272ec43420] [sp=0x7ffc2818b940]
 #6 epoll_wait + 0x1a [ip=0x7f272eb5c42a] [sp=0x7ffc2818bfd8]
 #7 epoll_event_loop + 0xd2 [ip=0x7f272f233e96] [sp=0x7ffc2818bfe0]
 #8 epoll_event_loop_once + 0x13c [ip=0x7f272f234864]
[sp=0x7ffc2818c040]
 #9 std_event_loop_once + 0x60 [ip=0x7f272f230f1d] [sp=0x7ffc2818c090]
 #10 _tevent_loop_once + 0x126 [ip=0x7f272f2278c8] [sp=0x7ffc2818c0d0]
 #11 tevent_common_loop_wait + 0x29 [ip=0x7f272f227c32]
[sp=0x7ffc2818c110]
 #12 std_event_loop_wait + 0x60 [ip=0x7f272f230fc3] [sp=0x7ffc2818c140]
 #13 _tevent_loop_wait + 0x2f [ip=0x7f272f227cd9] [sp=0x7ffc2818c180]
 #14 binary_smbd_main + 0x12a5 [ip=0x55ee8b8c284e] [sp=0x7ffc2818c1a0]
 #15 main + 0x76 [ip=0x55ee8b8c2911] [sp=0x7ffc2818c480]
 #16 __libc_start_main + 0xf3 [ip=0x7f272ea61083] [sp=0x7ffc2818c4b0]
 #17 _start + 0x2e [ip=0x55ee8b8bff4e] [sp=0x7ffc2818c580]

The reason for the current order is apparently IA-64, which is pretty
dead now, and perhaps has been fixed anyway.

Does anybody know/care about the IA-64 situation these days, or which
of libunwind or whatever backtrace_symbols() is likely to be on our
most important platforms would give?

I'm thinking we should:
 - invert the priority (prefer backtace_symbols if available)
 - only use/require libunwind (and have the option for --without-
libunwind) if backtrace_symbols is not present (typically in libc).

What do folks think?

Andrew Bartlett

-- 
Andrew Bartlett (he/him)       https://samba.org/~abartlet/
Samba Team Member (since 2001) https://samba.org
Samba Team Lead, Catalyst IT   https://catalyst.net.nz/services/samba

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