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Date: Thu, 26 Oct 2023 18:40:45 -0700
From: Peter Kasting <pkasting@...gle.com>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: vfprintf(..., "%lc", (wint_t)0) fails to output a \0
Sounds good. Thanks!
PK
On Thu, Oct 26, 2023, 6:38 PM Rich Felker <dalias@...c.org> wrote:
> On Thu, Oct 26, 2023 at 05:56:09PM -0700, Peter Kasting wrote:
> > (I'm not subscribed here, please CC me on any responses.)
> >
> > I believe* the following program will fail on musl:
> >
> > #include <assert.h>
> > #include <stdio.h>
> > #include <string.h>
> > #include <wchar.h>
> >
> > int main() {
> > char buf[16];
> > memset(buf, 1, 16);
> > int len = sprintf(buf, "%lc", (wint_t)0);
> > assert(len > 0);
> > assert(buf[0] == 0);
> > return 0;
> > }
> >
> > This should write a \0 to the buffer, but will write nothing.
> >
> > *I don't have direct access to a musl environment to compile and test the
> > code above, so this is speculation. I reproduced this bug indirectly via
> a
> > Linux Alpine test environment here at Google while trying to make changes
> > to Abseil's string-handling implementation.
> >
> > >From code inspection, the problem occurs at
> > https://git.musl-libc.org/cgit/musl/tree/src/stdio/vfprintf.c#n611. Here
> > the loop guard tests `*ws` to ensure it stops on null terminators.
> However,
> > when we fall through from handling this input case (of "%lc", 0), *ws ==
> > wc[0] == 0, so wctomb() is never called.
> >
> > The C99 spec here arguably allows this behavior, but I believe its intent
> > is to specify the behavior glibc's implementation exhibits (where a 0 is
> > written). Section 7.19.6.1.8 says regarding %lc, "...the wint_t argument
> is
> > converted as if by an ls conversion specification with no precision and
> an
> > argument that points to the initial element of a two-element array of
> > wchar_t, the first element containing the wint_t argument to the lc
> > conversion specification and the second a null wide character." And
> > regarding %ls, "...the argument shall be a pointer to the initial element
> > of an array of wchar_t type. Wide characters from the array are converted
> > to multibyte characters...up to and including a terminating null wide
> > character. The resulting multibyte characters are written up to (but not
> > including) the terminating null character (byte)." One could argue that
> the
> > first (zero) element in the array, being a 0, is "a terminating null wide
> > character" that should be converted to a multibyte character, but
> > subsequently not written (since the conversion will result in solely a
> > terminating null character). But it seems like the intent of the spec was
> > to say that a %lc argument is always converted and written, with the
> second
> > array element always treated as "the null terminator". I don't know if
> > there is further clarifying language/discussion in some mailing list or
> > archives somewhere.
>
> As I understand it, this was raised with the committee and since musl
> was found to be the only conforming implementation, they've opted to
> change the spec to what you expect was the intent. So we'll be
> changing this at some point to follow.
>
> Rich
>
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