[patch] Broken RLIMIT_FSIZE handling in ZFS

[Mateusz Guzik]

Hello,

vn_rlimit_fsize takes uio->uio_offset and uio->uio_resid into account
when determining whether given write would exceed RLIMIT_FSIZE.

When APPEND flag is specified, ZFS updates uio->uio_offset to point to the
end of file.

But this happens after a call to vn_rlimit_fsize, so vn_rlimit_fsize check
can be rendered ineffective by thread that opens some file with O_APPEND
and lseeks below RLIMIT_FSIZE before calling write.

This fixes the problem for me:
http://student.agh.edu.pl/~mjguzik/patches/zfs-rlimit-fsize.patch

Slightly modified testcase stolen from pr standards/164793:
http://student.agh.edu.pl/~mjguzik/patches/writelimit.c

Without the patch this testacase will just finish by producing 80000 bytes
file on ZFS.

On UFS it gives the following output:
failed when adding 27 bytes after 59994 bytes (error: File too large)

Same happens on ZFS with the patch.

-- 
Mateusz Guzik <mjguzik gmail.com>