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Date: Fri, 14 Sep 2018 12:24:09 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: string-backed FILEs mess
On Fri, Sep 14, 2018 at 11:52:27AM -0400, Rich Felker wrote:
> On Wed, Sep 12, 2018 at 01:18:24PM -0400, Rich Felker wrote:
> > 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.
>
> OK, a small breakthrough that makes this mess a lot simpler:
>
> The __intscan and __floatscan backends do not (and are not allowed to,
> but this should be documented and isn't) call any stdio functions on
> the fake FILE* passed to them except for the shgetc and shunget
> macros, defined as:
>
> #define shgetc(f) (((f)->rpos < (f)->shend) ? *(f)->rpos++ : __shgetc(f))
> #define shunget(f) ((f)->shend ? (void)(f)->rpos-- : (void)0)
>
> If the < is merely replaced by !=, which is functionally equivalent,
> then shend can be any type of sentinel pointer we want (e.g. (void*)-1
> or even just &localvar) to use the buffer as a string with no known
> length, and we have a guarantee that __shgetc is never called.
>
> I think this -1+2-byte change is an acceptable resolution to the issue
> for now.
Uhg, nope, mistake: they also use shcnt/shlim, which perform
arithmetic on f->shend. This fix might still be salvagable, but not
without significant additional work removing the dependency on invalid
arithmetic.
Rich
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