[PATCH] tempfile: avoid "ferror | fclose" trick
- From
Jeff King <peff@peff.net>
- Date
- Feb 16, 2017, 21:31 UTC
- Message-ID
- <20170216213140.xqw7gzjimhvg7tcm@sigill.intra.peff.net>
- In-Reply-To
- <20170216164359.k2ab7laqqvusfsm2@sigill.intra.peff.net>
On Thu, Feb 16, 2017 at 11:43:59AM -0500, Jeff King wrote:
Show 14 quoted lines
> On Thu, Feb 16, 2017 at 11:10:18AM +0100, Andreas Schwab wrote:
>
> > >> int xfclose(FILE *fp)
> > >> {
> > >> return ferror(fp) | fclose(fp);
> > >> }
> > >
> > > Yes, that's exactly what I had in mind (might be worth calling out the
> > > bitwise-OR, though, just to make it clear it's not a typo).
> >
> > Since the order of evaluation is unspecified, it would be better to
> > force sequencing ferror before fclose.
>
> Good point. Arguably the call in tempfile.c is buggy.Here's a fix.
I think close_tempfile() suffers from the same errno problem discussed earlier in this thread (i.e., that after calling it, you may get an error return with a random, unrelated errno value if ferror() failed but fclose() did not).
-- >8 -- Subject: [PATCH] tempfile: avoid "ferror | fclose" trick
The current code wants to record an error condition from either ferror() or fclose(), but makes sure that we always call both functions. So it can't use logical-OR "||", which would short-circuit when ferror() is true. Instead, it uses bitwise-OR "|" to evaluate both functions and set one or more bits in the "err" flag if they reported a failure.
Unlike logical-OR, though, bitwise-OR does not introduce a sequence point, and the order of evaluation for its operands is unspecified. So a compiler would be free to generate code which calls fclose() first, and then ferror() on the now-freed filehandle.
There's no indication that this has happened in practice, but let's write it out in a way that follows the standard.
Noticed-by: Andreas Schwab <schwab@linux-m68k.org> Signed-off-by: Jeff King <peff@peff.net> --- tempfile.c | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-)
diff --git a/tempfile.c b/tempfile.c index 2990c9242..ffcc27237 100644 --- a/tempfile.c +++ b/tempfile.c @@ -247,12 +247,8 @@ int close_tempfile(struct tempfile *tempfile) tempfile->fd = -1; if (fp) { tempfile->fp = NULL; - - /* - * Note: no short-circuiting here; we want to fclose() - * in any case! - */ - err = ferror(fp) | fclose(fp); + err = ferror(fp); + err |= fclose(fp); } else { err = close(fd); }
-- 2.12.0.rc1.559.gd292418bf