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Re: Building Emacs on WSL

From: Paul Eggert
Subject: Re: Building Emacs on WSL
Date: Tue, 20 Dec 2016 11:35:31 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

On 12/20/2016 04:40 AM, Fabrice Popineau wrote:

(gdb) run  --batch  --load loadup bootstrap
Starting program: /mnt/d/Source/emacs/build-emacs-ubuntu/src/temacs --batch --load loadup bootstrap
warning: Error disabling address space randomization: Succès

This is a warning from GDB, not from Emacs. GDB normally attempts to disable ASLR, to make debugging easier. Looking at the sequence of system calls in GDB 7.12, GDB calls 'personality' in some funky and undocumented ways, e.g., GDB assumes that errno is not changed when 'personality' succeeds.

Please compile and run the attached program under WSL, using the same flags you use to compile Emacs, and let me know what it does. On Fedora 24 x86-64, this program outputs:

personality (0xffffffff) returns 0x0, errno=0 (Success)
personality (pers | ADDR_NO_RANDOMIZE) returns 0x0, errno=0 (Success)
ADDR_NO_RANDOMIZE set successfully

When run under GDB, the same program outputs:

personality (0xffffffff) returns 0x40000, errno=0 (Success)

because GDB already disabled ASLR. You can look at GDB's file gdb/nat/linux-personality.c to see how GDB differs from Emacs in this area; perhaps there's something there that Emacs can use to figure out that it is running with a buggy kernel.

there is a simple fix:

That fix cannot be installed as-is since ASLR needs to be disabled on ordinary GNU/Linux platforms. It appears that WSL pretends to support the Linux 'personality' system call and its ADDR_NO_RANDOMIZE flag, and that this support does not actually work and makes Emacs crash.

Attachment: personality.c
Description: Text document

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