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| author | Christian Brauner <brauner@kernel.org> | 2024-09-10 10:16:39 +0200 |
|---|---|---|
| committer | Christian Brauner <brauner@kernel.org> | 2024-09-12 12:16:09 +0200 |
| commit | 2077006d4725c82c6e9612cec3a6c140921b067f (patch) | |
| tree | 626702ababcf6dbe391cd931e2ea63b798185ea3 /kernel/user.c | |
| parent | 698e7d1680544ef114203b0cf656faa0c1216ebc (diff) | |
| download | linux-2077006d4725c82c6e9612cec3a6c140921b067f.tar.gz linux-2077006d4725c82c6e9612cec3a6c140921b067f.tar.bz2 linux-2077006d4725c82c6e9612cec3a6c140921b067f.zip | |
uidgid: make sure we fit into one cacheline
When I expanded uidgid mappings I intended for a struct uid_gid_map to
fit into a single cacheline on x86 as they tend to be pretty
performance sensitive (idmapped mounts etc). But a 4 byte hole was added
that brought it over 64 bytes. Fix that by adding the static extent
array and the extent counter into a substruct. C's type punning for
unions guarantees that we can access ->nr_extents even if the last
written to member wasn't within the same object. This is also what we
rely on in struct_group() and friends. This of course relies on
non-strict aliasing which we don't do.
99) If the member used to read the contents of a union object is not the
same as the member last used to store a value in the object, the
appropriate part of the object representation of the value is
reinterpreted as an object representation in the new type as
described in 6.2.6 (a process sometimes called "type punning").
Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf
Link: https://lore.kernel.org/r/20240910-work-uid_gid_map-v1-1-e6bc761363ed@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Diffstat (limited to 'kernel/user.c')
| -rw-r--r-- | kernel/user.c | 6 |
1 files changed, 3 insertions, 3 deletions
diff --git a/kernel/user.c b/kernel/user.c index aa1162deafe4..f46b1d41163b 100644 --- a/kernel/user.c +++ b/kernel/user.c @@ -36,33 +36,33 @@ EXPORT_SYMBOL_GPL(init_binfmt_misc); */ struct user_namespace init_user_ns = { .uid_map = { - .nr_extents = 1, { .extent[0] = { .first = 0, .lower_first = 0, .count = 4294967295U, }, + .nr_extents = 1, }, }, .gid_map = { - .nr_extents = 1, { .extent[0] = { .first = 0, .lower_first = 0, .count = 4294967295U, }, + .nr_extents = 1, }, }, .projid_map = { - .nr_extents = 1, { .extent[0] = { .first = 0, .lower_first = 0, .count = 4294967295U, }, + .nr_extents = 1, }, }, .ns.count = REFCOUNT_INIT(3), |