Messages in this thread | | | Date | Tue, 23 Nov 2004 10:42:15 +0000 | From | Jamie Lokier <> | Subject | Re: [PATCH] Remove pointless <0 comparison for unsigned variable in fs/fcntl.c |
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Jesper Juhl wrote: > >> case F_SETSIG: > >> /* arg == 0 restores default behaviour. */ > >>- if (arg < 0 || arg > _NSIG) { > >>+ if (arg > _NSIG) { > >> break; > > > Let's find out.
The unusual thing about this function is that "arg" is really polymorphic, but given type "unsigned long" in the kernel. It is really a way to hold arbitrary values of any type.
Just look at the way it becomes "unsigned int" (dupfd) or "struct flock" (lock) or "long" (leases) or "int" (setown).
F_SETOWN is interesting because you really can pass a negative int argument and get a meaningful result, even though it's passed around as unsigned long for a little while.
Signal numbers are usually "int". The intended behaviour of fcntl(fd, F_SETSIG, sig) from userspace is that a negative sig returns EINVAL.
I.e. writing fcntl(fd, F_SETSIG, -1) in userspace will compile without any warnings. The intended behaviour is that a negative sig returns EINVAL. The kernel code illustrates that intention.
It isn't obvious that arg is unsigned long in this function, when reading the code. I had to scroll to the top of the function to check that this patch doesn't change its behaviour. For that reason I think the "< 0" test is useful, as it illustrates the intended behaviour and causes no harm.
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