Disable executable stack on Linux.

Disable executable stack on Linux.
BUG=29824
TEST=
william@penguin:~/src/chromium2/src$ out/Debug/chrome --user-data-dir=/tmp/foo&
[2] 28679
william@penguin:~/src/chromium2/src$ cat /proc/28679/maps | grep stack
7fff3fc4d000-7fff3fc63000 rw-p 00000000 00:00 0                          [stack]
william@penguin:~/src/chromium2/src$ eu-readelf -l out/Debug/chrome | grep STACK GNU_STACK
0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0x8

eu-readelf shows that the GNU_STACK segment indicates the stack should be RW (no X for execute).
`cat/proc/{pid}/maps | grep stack` shows that the stack mapping is indeed RW (if the dynamic linker encounters a shared library that needs an executable stack, it will globally enable executable stacks.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=43150

[Markus (顧孟勤), 2010-03-30 20:58:47]

LGTM

I would have expected that this was already the default behavior for "ld". But I
guess the toolchain developers never decided to make this the default. I wonder
why. Other than trampolines for nested functions (which pretty much nobody
uses), there is very little reason to have an executable stack.

[agl, 2010-03-30 21:33:29]

LGTM if it runs.

Here's a URL which is useless externally: http://b/issue?id=2372300

Discussed there in is the behaviour of ld, gold and what we needed to do to get
this flag going for some internal stuff.

[willchan no longer on Chromium, 2010-03-30 21:46:20]

On Tue, Mar 30, 2010 at 2:33 PM,  <agl@chromium.org> wrote:
> LGTM if it runs.
>
> Here's a URL which is useless externally: http://b/issue?id=2372300

Yes, I've already read that bug thread.  That's where I got the flag for ld.

>
> Discussed there in is the behaviour of ld, gold and what we needed to do to
> get
> this flag going for some internal stuff.
>
> http://codereview.chromium.org/1555002
>