linux-kernel - Re: x86-32: clean up rwsem inline asm statements

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	13 Jan 2010 19:27:08 -0500
From:	"George Spelvin" <linux@...izon.com>
To:	hpa@...or.com, linux@...izon.com
Cc:	linux-kernel@...r.kernel.org, schwab@...ux-m68k.org,
	torvalds@...ux-foundation.org
Subject: Re: x86-32: clean up rwsem inline asm statements

> There are a number of things that can be done better... for one thing,
> "+m" (sem->count) and "a" (sem) is just bloody wrong.  The right thing
> would be "a" (&sem->count) for proper robustness.

Actually, no.  The "+m" (sem->count) is telling GCC that sem->count is
updated; "a" (&sem->count) does *not* tell it to invalidate cached
copies of sem->count that it may have lying around.

However, we can't just use "+m" (sem->count) because GCC has a poor
grasp on the concept of atomic operations; as far as it is concerned,
it is exactly equivalent to copying the value into a stack slot, do the
operation there, and copy it back.

(It's much more likely to do that with "g" operand constraints, but it has
been known to point to a stack copy that it's made for some other reason.)

The current situation is, as far as I can remember the previous discussion
on the subject, the simplest way to explain to GCC just what it needs to do.

> There is no real point in being concerned about the type of immediates,
> because the immediate type isn't really used... it shows up as a literal
> in the assembly language.  However, if you're really concerned, the
> right thing to do is to do a cast in C, not playing games with the assembly.

Ah, right, sorry; I remembered having this problem, but it was with
register operands.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/