[Bug stdio/15589] freopen would close oldfd even though oldfd is same as newfd

["fweimer at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=15589

--- Comment #11 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Paul Eggert from comment #10)
> The usage that caused the bug is something like this, in a program that
> accepts a single arg FILE that may be "-" to denote standard input:
> 
>    if (strcmp (file, "-") != 0 && !freopen (file, "r", stdin))
>       error (EXIT_FAILURE, errno, file);

This looks far more reasonable.

> Here any error due to fd 0 being closed is irrelevant, as it's the first
> time the program has accessed stdin. This is a typical use case for freopen,
> and to some extent it's the reason freopen exists at all.

On the other hand, POSIX is quite clear that calling execve with the descriptor
0 not open for reading and 1/2 not open for writing is undefined.  If this
happens as the result of a shell command, it suggests a shell bug to me.

We could work around it in glibc by associating descriptors 0/1/2 with
/dev/null unconditionally, a not just for SUID programs, but this may impact
backwards compatibility for programs which never plan to use the standard
descriptors in the intended way.

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