Re: [PATCH] mmap.2: document new MAP_FIXED_NOREPLACE flag

Hi Jann,

On 04/11/2018 06:40 PM, Jann Horn wrote:

On Wed, Apr 11, 2018 at 6:36 PM, Michal Hocko [off-list ref] wrote:

quoted

On Wed 11-04-18 17:37:46, Jann Horn wrote:

quoted

On Wed, Apr 11, 2018 at 2:04 PM,  [off-list ref] wrote:

quoted

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.

Well, this was suggested by John. I think, nobody is objecting that
MAP_FIXED has legitimate usecases. The above text just follows up on
the previous section which emphasises the potential memory corruption
problems and it suggests that a new flag is safe with that regards.

If you have specific wording that would be better I am open for changes.

I guess I'd probably also want to change the previous text; so I
should probably send a followup patch once this one has landed.

Okay -- I'm ready to take that piece now. Please send me a patch!

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/