Re: [PATCH] mm: msync: require either MS_ASYNC or MS_SYNC
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From: Michael Kerrisk (man-pages) <hidden>
Date: 2014-04-01 19:32:18
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linux-mm, lkml
Richard, On 04/01/2014 08:25 PM, Richard Hansen wrote:
For the flags parameter, POSIX says "Either MS_ASYNC or MS_SYNC shall be specified, but not both." [1] There was already a test for the "both" condition. Add a test to ensure that the caller specified one of the flags; fail with EINVAL if neither are specified. Without this change, specifying neither is the same as specifying flags=MS_ASYNC because nothing in msync() is conditioned on the MS_ASYNC flag. This has not always been true,
I am curious (since such things should be documented)--when was it not true?
and there's no good reason to believe that this behavior would have persisted indefinitely. The msync(2) man page (as currently written in man-pages.git) is silent on the behavior if both flags are unset, so this change should not break an application written by somone who carefully reads the Linux man pages or the POSIX spec.
Sadly, people do not always carefully read man pages, so there remains the chance that a change like this will break applications. Aside from standards conformance, what do you see as the benefit of the change? Thanks, Michael
quoted hunk ↗ jump to hunk
[1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/msync.html Signed-off-by: Richard Hansen <rhansen-A08e6c8yq/Q@public.gmane.org> Reported-by: Greg Troxel <gdt-2FjktZCtrC/QT0dZR+AlfA@public.gmane.org> Reviewed-by: Greg Troxel <gdt-2FjktZCtrC/QT0dZR+AlfA@public.gmane.org> --- This is a resend of: http://article.gmane.org/gmane.linux.kernel/1554416 I didn't get any feedback from that submission, so I'm resending it without changes. mm/msync.c | 2 ++ 1 file changed, 2 insertions(+)diff --git a/mm/msync.c b/mm/msync.c index 632df45..472ad3e 100644 --- a/mm/msync.c +++ b/mm/msync.c@@ -42,6 +42,8 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t,len, int, flags) goto out; if ((flags & MS_ASYNC) && (flags & MS_SYNC)) goto out; + if (!(flags & (MS_ASYNC | MS_SYNC))) + goto out; error = -ENOMEM; len = (len + ~PAGE_MASK) & PAGE_MASK; end = start + len;
-- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/