1525803 - Use MAP_SHARED for read-only file mappings on MacOS and Android

Status: RESOLVED FIXED

(NSPR :: NSPR, defect, P1)

Description

[Jed Davis [:jld] (on leave until 08-31)]

Both MacOS and Android have shared memory implementations that need to be mapped with MAP_SHARED even for read-only use: POSIX shared memory on MacOS will return an error if mapped MAP_PRIVATE, and Android's ashmem will map zeroed memory instead of the memory object's contents in that case.

NSPR's PR_CreateFileMap could be used with these forms of shared memory by importing the fd, except that it tries to use MAP_PRIVATE for read-only mappings. There's already an #ifdefed workaround to use MAP_SHARED in that case, for an old version of OSF/1, which could be extended to other OSes. That shouldn't affect regular file mappings, because the difference between MAP_SHARED and MAP_PRIVATE is the result of writes to the mapping, which aren't allowed in the read-only case.

Comment 1

[Jed Davis [:jld] (on leave until 08-31)]

Attachment: bug1525803-nspr-shmem-hg0.diff

Comment 2

[Mike Hommey [:glandium]]

Comment on attachment 9042329 [details] [diff] [review]
bug1525803-nspr-shmem-hg0.diff

Review of attachment 9042329 [details] [diff] [review]:
-----------------------------------------------------------------

::: pr/src/md/unix/unix.c
@@ +3586,2 @@
>           */
>          fmap->md.flags = MAP_SHARED;

Considering the POSIX definition of MAP_SHARED vs. MAP_PRIVATE say they only make a difference for modifications to the mapped data by the calling process, and considering PROT_READ is never going to allow modifications, I'm almost tempted to say we could make it MAP_SHARED everywhere.

Anyways, I think it's fine like this.

Comment 3

[Jed Davis [:jld] (on leave until 08-31)]

I don't seem to have permission to push to NSPR.

Comment 4

[J.C. Jones [:jcj] (he/they)]

https://hg.mozilla.org/projects/nspr/rev/e61b118b47e6bda53067db0d3a755b4a6ed02c79