On Sat, 14 May 2022, Theo de Raadt wrote: > I worry a little about having libc use an undocumented mmap(2) flag. > About as much as using mquery, which is non-standard. > > Maybe __MAP_NOREPLACE should get documentation? __MAP_NOFAULT is in the > same situation. The behaviour of these flags should be documented > (set in stone), which may also discourage accidental behaviour changes > by kernel developers in the future?
To throw some spaghetti at the wall... Index: sys/mmap.2 =================================================================== RCS file: /data/src/openbsd/src/lib/libc/sys/mmap.2,v retrieving revision 1.68 diff -u -p -r1.68 mmap.2 --- sys/mmap.2 31 Mar 2022 17:27:16 -0000 1.68 +++ sys/mmap.2 14 May 2022 19:06:00 -0000 @@ -58,8 +58,19 @@ The argument describes the address where the system should place the mapping. If the .Dv MAP_FIXED -flag is specified, the allocation will happen at the specified address, +flag is specified and the +.Dv __MAP_NOREPLACE +flag is not specified, +the allocation will happen at the specified address, replacing any previously established mappings in its range. +If both the +.Dv MAP_FIXED +and +.Dv __MAP_NOREPLACE +flags are specified, +the allocation will happen at the specified address, +failing with no effect if any previously established mappings are +in its range. Otherwise, the mapping will be placed at the available spot at .Fa addr ; failing that it will be placed "close by". @@ -153,6 +164,17 @@ mappings) must be multiples of the page size. Existing mappings in the address range will be replaced. Use of this option is discouraged. +.It Dv __MAP_NOFAULT +Indicate that access to pages that are not backed by the mapped +file or device will be replaced by zero-filled anonymous pages. +This flag must not be used with an anonymous mapping. +.It Dv __MAP_NOREPLACE +Indicates that a +.Dv MAP_FIXED +mapping should fail if it would require replacing any existing +mappings. +This flag must be used in combination with +.Dv MAP_FIXED . .It Dv MAP_STACK Indicate that the mapping is used as a stack. This flag must be used in combination with @@ -278,6 +300,12 @@ device does not support memory mapping. The allocation .Fa len was 0. +.It Bq Er EINVAL +.Dv __MAP_NOFAULT +was specified for an anonymous mapping or +.Dv __MAP_NOREPLACE +was specified without +.Dv MAP_FIXED . .It Bq Er ENOMEM .Dv MAP_FIXED was specified and the @@ -323,6 +351,14 @@ A fully functional system call first appeared in SunOS 4.0 and has been available since .Bx 4.3 Net/2 . +The +.Dv __MAP_NOFAULT +flag appeared in +.Ox 5.7 . +The +.Dv __MAP_NOREPLACE +flag appeared in +.Ox 5.3 . .Sh CAVEATS .St -p1003.1-2008 specifies that references to pages beyond the end of a mapped object
