On Wed, Apr 11, 2018 at 6:36 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: > On Wed 11-04-18 17:37:46, Jann Horn wrote: >> On Wed, Apr 11, 2018 at 2:04 PM, <mhocko@xxxxxxxxxx> wrote: >> > From: Michal Hocko <mhocko@xxxxxxxx> >> > >> > 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 <jhubbard@xxxxxxxxxx>] >> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> >> > --- >> > 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. >> > +.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. > > sure, no objection from me. > > -- > Michal Hocko > SUSE Labs
