Re: [PATCH] mmap.2: document new MAP_FIXED_NOREPLACE flag
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From: Jann Horn <jannh@google.com>
Date: 2018-04-11 15:38:09
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linux-mm, lkml
On Wed, Apr 11, 2018 at 2:04 PM, [off-list ref] wrote:
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From: Michal Hocko <mhocko@suse.com> 4.17+ kernels offer a new MAP_FIXED_NOREPLACE flag which allows the caller to atomicaly probe for a given address range. [wording heavily updated by John Hubbard [off-list ref]] Signed-off-by: Michal Hocko <mhocko@suse.com> --- Hi, Andrew's sent the MAP_FIXED_NOREPLACE to Linus for the upcoming merge window. So here we go with the man page update. man2/mmap.2 | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+)diff --git a/man2/mmap.2 b/man2/mmap.2 index ea64eb8f0dcc..f702f3e4eba2 100644 --- a/man2/mmap.2 +++ b/man2/mmap.2@@ -261,6 +261,27 @@ Examples include and the PAM libraries .UR http://www.linux-pam.org .UE . +Newer kernels +(Linux 4.17 and later) have a +.B MAP_FIXED_NOREPLACE +option that avoids the corruption problem; if available, MAP_FIXED_NOREPLACE +should be preferred over MAP_FIXED.
This still looks wrong to me. There are legitimate uses for MAP_FIXED, and for most users of MAP_FIXED that I'm aware of, MAP_FIXED_NOREPLACE wouldn't work while MAP_FIXED works perfectly well. MAP_FIXED is for when you have already reserved the targeted memory area using another VMA; MAP_FIXED_NOREPLACE is for when you haven't. Please don't make it sound as if MAP_FIXED is always wrong.
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+.TP +.BR MAP_FIXED_NOREPLACE " (since Linux 4.17)" +Similar to MAP_FIXED with respect to the +.I +addr +enforcement, but different in that MAP_FIXED_NOREPLACE never clobbers a pre-existing +mapped range. If the requested range would collide with an existing +mapping, then this call fails with +.B EEXIST. +This flag can therefore be used as a way to atomically (with respect to other +threads) attempt to map an address range: one thread will succeed; all others +will report failure. Please note that older kernels which do not recognize this +flag will typically (upon detecting a collision with a pre-existing mapping) +fall back to a "non-MAP_FIXED" type of behavior: they will return an address that +is different than the requested one. Therefore, backward-compatible software +should check the returned address against the requested address. .TP .B MAP_GROWSDOWN This flag is used for stacks.@@ -487,6 +508,12 @@ is not a valid file descriptor (and .B MAP_ANONYMOUS was not set). .TP +.B EEXIST +range covered by +.IR addr , +.IR length +is clashing with an existing mapping.
Maybe add something like ", and MAP_FIXED_NOREPLACE was specified"? I think most manpages explicitly document which error conditions can be triggered by which flags.