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Re: CVS commit: src/bin/sh

On Wed, 15 Oct 2014, Christos Zoulas wrote:
Modified Files:
	src/bin/sh: redir.c

Log Message:
PR/48201: Miwa Susumu: Fix set -C (no clobber) for POSIX; from FreeBSD
Can't use O_EXCL because of device nodes; also truncate.

There are some TOCTOU races in this code, where something about the file could change in between the stat() and the open().

Some ideas:

1. Keep the new code, with its races, but also verify that st_dev and st_ino values remain unchanged between the stat() before opening the file, and fstat() after opening the file.

2. Try open() with O_EXCL first, and fall back to racy code with stat() only if the first open(O_EXCL) fails. Also use fstat() to check that st_dev/st_ino do not change.

3. Invent one or more open(2) flags, such as O_SPECIAL for "must be a device or other special file, must not be a plain file or a directory". First try open(O_EXCL), and if that fails then try open(O_SPECIAL).

--apb (Alan Barrett)

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