Re: [PATCH] tmpfs/ramfs: Let memfd_create() work on nommu

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

On Sat, 23 May 2026 22:04:45 +0900 Daniel Palmer <daniel@thingy.jp> wrote:

> 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.
> 

Old patch, older code.   Thanks.

> Really not sure if this is correct. It works for me but on
> nommu a lot of things just work because there is no MMU to
> shout about badness.
> 
> Maybe shashiko will say this is a dumb patch and explain
> the proper fix. :)

Sashiko had no complaints but it seems to have found some unrelated
bugs:

	https://sashiko.dev/#/patchset/20260523130445.1101818-1-daniel@thingy.jp

(tmpfs calls "ramfs_nommu" code.  Who knew?)

> Also I had almost no idea who to send this to from the output
> of get_maintainer.pl.

I suspect most would prefer it's someone else, but this is viro/brauner
stuff, if anyone)

> --- a/fs/ramfs/file-nommu.c
> +++ b/fs/ramfs/file-nommu.c
> @@ -69,6 +69,9 @@ int ramfs_nommu_expand_for_mapping(struct inode *inode, size_t newsize)
>  	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;

Oh well, thanks, I'll toss it onto the pile and see what happens.