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Message-ID: <20190221172755.799b0b63@inria.fr>
Date: Thu, 21 Feb 2019 17:27:55 +0100
From: Jens Gustedt <jens.gustedt@...ia.fr>
Cc: musl@...ts.openwall.com
Subject: Re: Stdio resource usage
Hello,
On Thu, 21 Feb 2019 17:09:37 +0100 Markus Wichmann <nullplan@....net>
wrote:
> On Wed, Feb 20, 2019 at 02:24:23PM -0500, Rich Felker wrote:
> So, what are our choices?
>
> - Heap allocation: But that can fail. Now, printf() is actually
> allowed to fail, but no-one expects it to. I would expect such
> behavior to be problematic at best.
> - Static allocation: Without synchronization this won't be
> thread-safe, with synchronization it won't be re-entrant. Now, as far
> as I could see, the printf() family is actually not required to be
> re-entrant (e.g. signal-safety(7) fails to list any of them), but I
> have seen sprintf() in signal handlers in the wild (well, exception
> handlers, really).
> - Thread-local static allocation: Which is always a hassle in libc,
> and does not take care of re-entrancy. It would only solve the
> thread-safety issue.
> - As-needed stack allocation (e.g. alloca()): This fails to prevent
> the worst case allocation, though it would make the average allocation
> more bearable. But I don't know if especially clever compilers like
> clang wouldn't optimize this stuff away, and we'd be back to square
> one.
Perhaps the latter, but maybe with VLA? Unfortunately these techniques
have no reliable error detection mechanism.
For the static allocation strategy one could try to implement
something like a "bounded" stack, that is two or three versions of the
data in a array, protected by a lock and a counter, such that at least
one level of signal handler could still use it. But this is probably a
bit tedious to implement.
Jens
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