Clarify that MAP_FIXED is appropriate if the specified address range has been reserved using an existing mapping, but shouldn't be used otherwise.
Signed-off-by: Jann Horn <ja...@google.com> --- man2/mmap.2 | 19 +++++++++++-------- 1 file changed, 11 insertions(+), 8 deletions(-) diff --git a/man2/mmap.2 b/man2/mmap.2 index bef8b4432..80c9ec285 100644 --- a/man2/mmap.2 +++ b/man2/mmap.2 @@ -253,8 +253,9 @@ Software that aspires to be portable should use this option with care, keeping in mind that the exact layout of a process's memory mappings is allowed to change significantly between kernel versions, C library versions, and operating system releases. -Furthermore, this option is extremely hazardous (when used on its own), -because it forcibly removes preexisting mappings, +This option should only be used when the specified memory region has +already been reserved using another mapping; otherwise, it is extremely +hazardous because it forcibly removes preexisting mappings, making it easy for a multithreaded process to corrupt its own address space. .IP For example, suppose that thread A looks through @@ -284,13 +285,15 @@ and the PAM libraries .UR http://www.linux-pam.org .UE . .IP -Newer kernels -(Linux 4.17 and later) have a +For cases in which the specified memory region has not been reserved using an +existing mapping, newer kernels (Linux 4.17 and later) provide an option .B MAP_FIXED_NOREPLACE -option that avoids the corruption problem; if available, -.B MAP_FIXED_NOREPLACE -should be preferred over -.BR MAP_FIXED . +that should be used instead; older kernels require the caller to use +.I addr +as a hint (without +.BR MAP_FIXED ) +and take appropriate action if the kernel places the new mapping at a +different address. .TP .BR MAP_FIXED_NOREPLACE " (since Linux 4.17)" .\" commit a4ff8e8620d3f4f50ac4b41e8067b7d395056843 -- 126.96.36.1994.g0c8726318c-goog