+ tmpfs-ramfs-let-memfd_create-work-on-nommu.patch added to mm-nonmm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: tmpfs/ramfs: let memfd_create() work on nommu
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     tmpfs-ramfs-let-memfd_create-work-on-nommu.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/tmpfs-ramfs-let-memfd_create-work-on-nommu.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days

------------------------------------------------------
From: Daniel Palmer <daniel@thingy.jp>
Subject: tmpfs/ramfs: let memfd_create() work on nommu
Date: Sat, 23 May 2026 22:04:45 +0900

Currently trying to use memfd_create() on nommu returns an error with
errno set to EFBIG.  The manpage memfd_create() doesn't have EFBIG as a
possible error value.

Doing some digging this is coming from 0 getting passed as newsize to
ramfs_nommu_expand_for_mapping() and that getting into get_order() and
there "The result is undefined if the size is 0".

Whatever comes out of get_order() is then used in the following logic and
that results in the EFBIG that causes the syscall to fail and the errno in
userspace.

If newsize is 0 there is nothing to do so just return.

Roughly tested on m68k nommu by creating a process, creating an memfd,
forking another process, mmap()ing the memfd in the child, writing into
the mapping, then mmap()ing in the parent and checking that the right data
is there.

Link: https://lore.kernel.org/20260523130445.1101818-1-daniel@thingy.jp
Signed-off-by: Daniel Palmer <daniel@thingy.jp>
Acked-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: "Liam R. Howlett" <liam@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/ramfs/file-nommu.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/fs/ramfs/file-nommu.c~tmpfs-ramfs-let-memfd_create-work-on-nommu
+++ a/fs/ramfs/file-nommu.c
@@ -69,6 +69,9 @@ int ramfs_nommu_expand_for_mapping(struc
 	gfp_t gfp = mapping_gfp_mask(inode->i_mapping);
 
 	/* make various checks */
+	if (!newsize)
+		return 0;
+
 	order = get_order(newsize);
 	if (unlikely(order > MAX_PAGE_ORDER))
 		return -EFBIG;
_

Patches currently in -mm which might be from daniel@thingy.jp are

tmpfs-ramfs-let-memfd_create-work-on-nommu.patch