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Message-ID: <20120210074002.GA13559@openwall.com>
Date: Fri, 10 Feb 2012 11:40:02 +0400
From: Solar Designer <solar@...nwall.com>
To: musl@...ts.openwall.com
Subject: Re: tough choice on thread pointer initialization issue
Hi Rich,
Thank you for posting this!
On Thu, Feb 09, 2012 at 09:58:25PM -0500, Rich Felker wrote:
> approach 1: hack the signal-return "restore" function to save the
> current thread register value into the struct sigcontext before
> calling SYS_sigreturn, so that it will be preserved when the
> interrupted code resumes.
>
> pros: minimal costs, never adds any syscalls versus current musl.
>
> cons: ugly hack, and gdb does not like non-canonical sigreturn
> functions (it refuses to work when the instruction pointer is at
> them).
>
>
> approach 2: call pthread_self() from sigaction(). this will ensure
> that a signal handler never runs prior to the thread pointer being
> initialized.
>
> pros: minimal code changes, and avoids adding syscalls except for
> programs that use signals but not threads.
>
> cons: adds a syscall, and links unnecessary thread code when static
> linking, in any program that uses signal handlers.
I think another con of the two approaches above is that they'll fail if
a program sets up a signal handler in a way bypassing musl (and other
prerequisites of the problem are met as well, as you described them).
Indeed, this makes it even more of a special case, but it's still legal
(or not? that's a musl policy thing I guess).
> approach 3: always initialize the thread pointer from
> __libc_start_main (befoe main runs). (this is the glibc approach)
...
> before i make a decision, i'd like to hear if anyone from the
> community has strong opinions one way or the other. i've almost ruled
> out approach #1 and i'm leaning towards #3, with the idea that
> simplicity is worth more than a couple trivial syscalls.
Not a strong opinion, but how about:
approach 4: initialize the thread pointer register to zero at program
startup. Then before its use, just check it for non-zero instead of
checking a global flag. (I presume that you're doing the latter now,
based on your description of the problem.)
Alexander
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