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Date: Wed, 12 Sep 2018 13:18:24 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: string-backed FILEs mess
On Wed, Sep 12, 2018 at 12:33:45PM -0400, Rich Felker wrote:
> OK, I've been properly initializing the FILE rather than leaving it
> uninitialized except for the important fields like the old code did.
> Changing that, it's 1.44s with step 8, 1.60s with step 24. I also
> confirmed that this version of the code is almost as fast as the
> existing code with the memchr removed (just assuming it can read
> ahead).
Uhg, the source of the "almost" here makes me even more convinced the
current code must go. Part of the reason it's not as fast was that I
was still setting f.read=__string_read, which requires (this is on
i386, 32-bit) setting up the GOT pointer.
What was the old code doing? f.read was uninitialized. But the new
code crashes in that case when hitting the end of the string. Why
doesn't the old code crash? Because f.rend is set way past the end of
the string and never reached. If it were reached:
1. The shgetc macro calls the __shgetc function.
2. The __shgetc function calls __uflow.
3. __uflow calls __toread.
4. __toread inspects uninitialized f->wpos/f->wbase fields and,
depending on the values it sees, calls f->write, which is also
uninitialized.
5. If it gets past that, next __uflow calls the uninitialized f->read.
The fact that any of this works at all is a fragile shit-show, and
completely depends on __intscan/__floatscan just using (via shgetc) a
tiny undocumented subset of the FILE structure and a tiny undocumented
subset of the stdio interfaces on it.
Really the existing code is just a poor substitute for having an
abstraction for windowed string iteration, using the stdio FILE
structure in a way that also works with real FILEs. It's clever, but
this kind of clever is not a good thing.
I'm still not sure what the right way forward is, though.
Rich
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