linux-kernel - Re: [PATCH] signalfd: retrieve multiple signals with one read() call

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Message-Id: <20070520212627.398ffd23.akpm@linux-foundation.org>
Date:	Sun, 20 May 2007 21:26:27 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Davi Arnaut <davi@...ent.com.br>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] signalfd: retrieve multiple signals with one read()
 call

On Sun, 20 May 2007 21:14:38 -0700 (PDT) Davide Libenzi <davidel@...ilserver.org> wrote:

> On Sun, 20 May 2007, Andrew Morton wrote:
> 
> > If 'count' is not a multiple of sizeof(struct signalfd_siginfo)), the read()
> > will return the next smallest multiple of `count'.
> > 
> > That is, unless `count' happens to be less than 1*sizeof(struct
> > signalfd_siginfo)), in which case we return -EINVAL.
> > 
> > This seems inconsistent.
> 
> I think it fits the rule "buffer must be big enough for at least one sigingo".
> We use the special return 0; as indicator that the process we were 
> monitoring signals, detached the sighand.
> 

hm.  Kernel violates proper read() semantics in many places.  Looks like we
just did it again.

> 
> > Also, I'm desperately hunting for the place where we zero out that local
> > siginfo_t, and I ain't finding it.  Someone please convince me that we're
> > not leaking bits of kernel memory out to userspace in that thing.
> 
> Hmm, __clear_user()?

oic, yes, that thing.  Usually we'd zero out the on-stack struct, assemble
it then copy out the whole thing.  I guess doing it the way you have saves
a few instructions.  But it's the cache hit against *uinfo which will have
most of the cost, and we can't do anything about that.

Unless we just remove the __clear_user() altogether.  Who said that "Unused
memebers should be zero"?

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