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Date: Sun, 24 Jan 2021 13:55:45 -0500
From: Rich Felker <>
To: Alexander Monakov <>
Cc:, Andrew Rogers <>
Subject: Re: Potential DL_NOMMU_SUPPORT bug.

On Sun, Jan 24, 2021 at 09:48:11PM +0300, Alexander Monakov wrote:
> > > sdcard [pseudo-]partition is usually mounted noexec, so mmap with PROT_EXEC
> > > should fail.
> > 
> > Uhg, that makes no sense. Does it enforce that even for MAP_PRIVATE,
> > which should semantically be equivalent to just making anon memory
> > with the requested permissions and copying the file contents into it??
> I think it makes sense: isn't the entire point of 'noexec' that a user
> who has write access only to noexec filesystems will not be able to run
> arbitrary binary code (assuming the already-present binaries are not
> cooperative, unlike musl with the above patch would be)? Enforcing
> noexec for MAP_PRIVATE ensures the users can not trivially side-step
> noexec by invoking (without extra checks on side).

I always viewed noexec (as opposed to something like nosuid) as a
non-security-boundary, a sort of soft block for mounting filesystems
that you don't want to execute programs from, for example a disk image
known to contain malware that you're analyzing or a flash drive not
expected to contain meaningful executable data but where all files
would appear as +x due to FAT limitations. The expectation is that it
can be bypassed. With a "restricted shell" type environment (very
careful selection of what programs are present), it can plausibly be
turned into a (very fragile) security boundary, but I didn't expect
the kernel to be making weird rules to facilitate that.

In any case, it seems that's how it is, and inability to dlopen (or
LD_LIBRARY_PATH+DT_NEEDED or whatnot) from a noexec mount is


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