Re: MAP_FIXED leaving holes on failure is very difficult to recover from and can even be a security hole

[Demi Marie Obenour]

On 11/5/26 10:11, Lorenzo Stoakes (ARM) wrote:
> [ please check kernel mails in MAINTAINERS - my and Liam's at least were
>   incorrect, which makes it easy to miss. Fixed those up. ]

I thought I did that, but obviously I didn't do it properly.

> Thanks for the report.
> 
> In general linux system calls do not guarantee compound operation
> atomicity, and any assumption based on that is incorrect. If you see an
> error - the operation failed, perhaps partially.
> 
> Further, a failed MAP_FIXED leaving gaps was the case for most of the> history of linux. Avoiding that is infeasible.
> 
> I advise you to review the NOTES section
> https://man7.org/linux/man-pages/man2/mmap.2.html on MAP_FIXED - your
> suggested use of it is not recommended.

Does that mean that MAP_FIXED should be deprecated?  It seems that the
safe use-cases can all use MAP_FIXED_NOREPLACE, and that the use-cases
that cannot be served by MAP_FIXED_NOREPLACE cannot be served by current
Linux.

Specifically, if a MAP_FIXED mmap() fails,  the mapping might either
still be there (in which case one _must_ call munmap() to cleanup)
or it might not be there (in which case one _must not_ call munmap()).
There's no race-free way to tell which is the case.  This means that
MAP_FIXED is unusable in multithreaded programs, unless one is okay
with leaking address space.

Are there cases, such as anonymous mappings, that cannot leave holes?
I believe there are some programs that depend on this to handle
SIGBUS faults from truncated mapped files.  If even that can leave
a hole, the only thread-safe alternative would be longjmp() out of
the SIGBUS handler.

An alternative behavior would be to unmap the pages, but keep the
address space reserved, as if the mapping was a PROT_NONE anonymous one.
That would avoid the "it may or may not have been freed" situation,
as it would be safe to retry the mapping or call munmap().
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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