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Date: Sat, 20 Aug 2022 10:49:43 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: Rich Felker <dalias@...c.org>
Cc: Érico Nogueira <ericonr@...root.org>,
musl@...ts.openwall.com
Subject: Re: [PATCH] remove extraneous syscall from fopen(3)
* Rich Felker <dalias@...c.org> [2022-08-15 14:16:09 -0400]:
> On Mon, Aug 15, 2022 at 02:58:40PM -0300, Érico Nogueira wrote:
> > On Mon Aug 15, 2022 at 2:54 PM -03, Rich Felker wrote:
> > > On Mon, Aug 15, 2022 at 02:50:21PM -0300, Érico Nogueira wrote:
> > > > the __fdopen() call afterwards will set the close-on-exec flag with the
> > > > same syscall if "e" was specified in mode
> > > > ---
> > > > src/stdio/fopen.c | 2 --
> > > > 1 file changed, 2 deletions(-)
> > > >
> > > > diff --git a/src/stdio/fopen.c b/src/stdio/fopen.c
> > > > index e1b91e12..22b72edf 100644
> > > > --- a/src/stdio/fopen.c
> > > > +++ b/src/stdio/fopen.c
> > > > @@ -20,8 +20,6 @@ FILE *fopen(const char *restrict filename, const char *restrict mode)
> > > >
> > > > fd = sys_open(filename, flags, 0666);
> > > > if (fd < 0) return 0;
> > > > - if (flags & O_CLOEXEC)
> > > > - __syscall(SYS_fcntl, fd, F_SETFD, FD_CLOEXEC);
> > > >
> > > > f = __fdopen(fd, mode);
> > > > if (f) return f;
> > > > --
> > > > 2.37.2
> > >
> > > See commit 7765706c0584ed4a30e0b7a3ada742e490ef02b0
> >
> > If the relevant part of that commit is that the flag is added
> > immediately after, would moving the SYS_fcntl call in __fdopen to the
> > top of the functon be acceptable?
>
> Oh, I missed that it also happens in __fdopen from the 'e' being
> present, and misunderstood your patch as just removing the fallback
> entirely.
>
> No, it's not acceptable to move the fcntl in __fdopen above the malloc
> because it would make fdopen modify the fd status on failure. I guess
shouldn't fopen close fd on fdopen failure?
> it's questionable whether we care "how soon" after the open it happens
> -- either way this is not a thread-safe fallback precluding fd leak on
> old/broken kernels. But since malloc may be application-provided,
> failure to set it before the malloc like we're doing now would be a
> "worse behavior" in some sense, exposing the incorrect fd state to a
> non-multithreaded application. So I'm not sure if it's a good idea to
> change this or not. Do you have reason to believe it's affecting
> performance in real-world usage?
>
> Rich
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