On 04/12/2018 05:39 PM, Jann Horn wrote:
> 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.
> For example, suppose that thread A looks through
> @@ -284,13 +285,15 @@ and the PAM libraries
> .UR http://www.linux-pam.org
> .UE .
> -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.
> .BR MAP_FIXED_NOREPLACE " (since Linux 4.17)"
> .\" commit a4ff8e8620d3f4f50ac4b41e8067b7d395056843
Thanks! Nice patch! Applied.
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