[PATCH v2] man/man2const/F_{ADD,GET}_SEALS.2const: document F_SEAL_EXEC

From: "Pratyush Yadav (Google)" <[email protected]>

F_SEAL_EXEC was added in Linux v6.3. It blocks changing of the exec bits
once added. Document it.

Signed-off-by: Pratyush Yadav (Google) <[email protected]>
---

Notes:
    I discovered this was missing when working on [0]. I had to look at the
    code to figure out how it was supposed to behave.
    
    Changes in v2:
    - Re-write the documentation by hand.
    
    [0] https://lore.kernel.org/linux-mm/[email protected]/

 man/man2const/F_GET_SEALS.2const | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/man/man2const/F_GET_SEALS.2const b/man/man2const/F_GET_SEALS.2const
index 175025c10..f41e1748a 100644
--- a/man/man2const/F_GET_SEALS.2const
+++ b/man/man2const/F_GET_SEALS.2const
@@ -176,6 +176,25 @@ will fail with
 Using this seal,
 one process can create a memory buffer that it can continue to modify
 while sharing that buffer on a "read-only" basis with other processes.
+.TP
+.BR F_SEAL_EXEC " (since Linux 6.3)"
+If this seal is set, the execute mode bits of the file cannot be modified.
+Attempting to change the execute mode bits via
+.BR fchmod (2)
+or similar will fail with
+.BR EPERM .
+This results in a memfd that is either permanently executable or
+permanently un-executable.
+.IP
+Adding this seal implicitly adds
+.BR F_SEAL_GROW ,
+.BR F_SEAL_SHRINK ,
+.BR F_SEAL_WRITE ,
+and
+.BR F_SEAL_FUTURE_WRITE .
+This ensures that the executable code is not writeable.
+All the pre-requisites to add the implied seals must be met to successfully add
+.BR F_SEAL_EXEC .
 .SH RETURN VALUE
 .TP
 .B F_GET_SEALS

base-commit: 9db8ca91f920b9aba40ed68de6b8da0ca9dbefaa
-- 
2.54.0.1013.g208068f2d8-goog