From: Paul Eggert <eggert@cs.ucla.edu>
To: Thomas Fitzsimmons <fitzsim@fitzsim.org>
Cc: 33174@debbugs.gnu.org
Subject: bug#33174: 27.0.50; Dump fails on GNU/Linux ppc64le
Date: Mon, 29 Oct 2018 22:58:19 -0700 [thread overview]
Message-ID: <9fbbce6a-ca72-e4e2-1456-49e146542896@cs.ucla.edu> (raw)
In-Reply-To: <m35zxkcfwf.fsf@fitzsim.org>
Thomas Fitzsimmons wrote:
> BTW, let me know if you don't think it's useful to debug this further.
> I'm OK just disabling randomization when I build Emacs for the time
> being and waiting until the portable dumper work lands, but I'm happy to
> continue if you think it will lead to a general fix.
It's not clear when the portable dumper will land; it might not ever land,
unfortunately. So I would like to work on bug#33174 a bit longer, if only so
that we can put something intelligible into the PROBLEMS file.
> It seems like it's crashing when trying to memcpy over the BSS area, on
> this line in unexelf.c (see below):
By the time the memcpy is run the damage has already been done: the memory
layout is messed up and we can't fix that simply by passing different arguments
to memcpy. We have to prevent the memory layout from being messed up in the
first place by disabling undesirable address space layout randomization and
doing this very early in execution.
The key question for me is in this set of system calls:
> 58215 personality(0xffffffff) = 0 (PER_LINUX)
> 58215 personality(PER_LINUX|ADDR_NO_RANDOMIZE) = 0 (PER_LINUX)
> 58215 personality(0xffffffff) = 0x40000 (PER_LINUX|ADDR_NO_RANDOMIZE)
> 58215 brk(NULL) = 0x27070000
> 58215 dup2(0, 0) = 0
> 58215 dup2(1, 1) = 1
> 58215 dup2(2, 2) = 2
Surely the call to disable_address_randomization () must have returned true, but
can you verify that, either via GDB or (shudder) by inserting print statements?
Also, the call from 'main' to getenv ("EMACS_HEAP_EXEC") must have returned
NULL. Can you also verify this?
And it appears that 'main' must have called xputenv ("EMACS_HEAP_EXEC=true") and
execvp (argv[0], argv). But how can this be, since there's no execve syscall?
This is the heart of the mystery, and we can find out more about it by using GDB
to put breakpoints on 'personality', 'getenv', 'xputenv' and/or 'execvp' and
seeing what's going on. Something like this, perhaps:
$ gdb temacs
(gdb) set disable-randomization off
(gdb) b personality
(gdb) b getenv
(gdb) b xputenv
(gdb) b execvp
(gdb) r --batch --load loadup bootstrap
and seeing which of these functions get executed in what order, and what they
return.
next prev parent reply other threads:[~2018-10-30 5:58 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-27 10:35 bug#33174: 27.0.50; Dump fails on GNU/Linux ppc64le Thomas Fitzsimmons
2018-10-27 13:16 ` Thomas Fitzsimmons
2018-10-27 20:26 ` Paul Eggert
2018-10-28 8:05 ` Paul Eggert
2018-10-28 12:56 ` Thomas Fitzsimmons
2018-10-28 19:22 ` Paul Eggert
2018-10-30 1:28 ` Thomas Fitzsimmons
2018-10-30 2:16 ` Paul Eggert
2018-10-29 1:47 ` Thomas Fitzsimmons
2018-10-29 7:22 ` Paul Eggert
2018-10-30 2:44 ` Thomas Fitzsimmons
2018-10-30 5:58 ` Paul Eggert [this message]
2018-10-30 9:30 ` Thomas Fitzsimmons
2018-11-12 6:39 ` Paul Eggert
2018-11-12 17:24 ` Andy Moreton
2018-11-13 2:29 ` Thomas Fitzsimmons
2018-11-13 14:42 ` Paul Eggert
2018-11-19 3:14 ` Thomas Fitzsimmons
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