Re: [PATCH v2] coredump: add %i/%I in core_pattern to report the tid of the crashed thread

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 22 Sep 2014 19:15:25 +0200 Oleg Nesterov <oleg@redhat.com> wrote:

> format_corename() can only pass the leader's pid to the core handler, but
> there is no simple way to figure out which thread originated the coredump.
> 
> As Jan explains, this also means that there is no simple way to create the
> backtrace of the crashed process:
> 
> As programs are mostly compiled with implicit gcc -fomit-frame-pointer one
> needs program's .eh_frame section (equivalently PT_GNU_EH_FRAME segment) or
> .debug_frame section. .debug_frame usually is present only in separate debug
> info files usually not even installed on the system.  While .eh_frame is a
> part of the executable/library (and it is even always mapped for C++
> exceptions unwinding) it no longer has to be present anywhere on the disk
> as the program could be upgraded in the meantime and the running instance
> has its executable file already unlinked from disk.
> 
> One possibility is to echo 0x3f >/proc/*/coredump_filter and dump all the
> file-backed memory including the executable's .eh_frame section. But that
> can create huge core files, for example even due to mmapped data files.
> 
> Other possibility would be to read .eh_frame from /proc/PID/mem at the
> core_pattern handler time of the core dump.  For the backtrace one needs to
> read the register state first which can be done from core_pattern handler:
> 
> 	ptrace(PTRACE_SEIZE, tid, 0, PTRACE_O_TRACEEXIT)
> 	close(0);    // close pipe fd to resume the sleeping dumper
> 	waitpid();   // should report EXIT
> 	PTRACE_GETREGS or other requests
> 
> The remaining problem is how to get the 'tid' value of the crashed thread.
> It could be read from the first NT_PRSTATUS note of the core file but that
> makes the core_pattern handler complicated.
> 
> Unfortunately %t is already used so this patch uses %i/%I.

Is any userspace actually going to use this?  If so, details?

Am wondering what is driving this change...