I'm having trouble squaring the docs in mount(8) [1]:
defaults
Use the default options: rw, suid, dev, exec, auto, nouser, and
async.
Note that the real set of all default mount options depends on
kernel and filesystem type. See the beginning of this section
for more details.
and fstab(5) [2]:
defaults
use default options: rw, suid, dev, exec, auto, nouser, and async.
with the implementation [3]:
{ "defaults", 0, 0 }, /* default options */
where the struct fields are [4]:
* @name: option name[=type] where type is printf-like type specifier")
* @id: option ID or MS_* flags (e.g MS_RDONLY)
* @mask: MNT_{NOMTAB,INVERT,...} mask
As far as I can tell, that makes 'defaults' a no-op. For example, it
doesn't clear MS_RDONLY:
# strace -o /tmp/trace mount -t tmpfs -o ro,defaults - /tmp/a
# grep 'mount(' /tmp/trace
mount("-", "/tmp/a", "tmpfs", MS_MGC_VAL|MS_RDONLY, NULL) = 0
But using ‘rw’ directly does:
# strace -o /tmp/trace mount -t tmpfs -o ro,rw - /tmp/a
# grep 'mount(' /tmp/trace
mount("-", "/tmp/a", "tmpfs", MS_MGC_VAL, NULL) = 0
And as you can see in those strace logs, the presence of ‘defaults’ in
userspace_opts_map means that it doesn't get passed through in
mount(2)'s ‘data’ argument either.
I can think of a few possibilities:
a. ‘defaults’ is supposed to be a no-op placeholder for the fourth
fstab field. We should drop it from examples that set any other
options (like [5]) and update the man entries [1,2] to remove the
references to ‘rw’, ‘suid’, etc. and replace them with markers on
the inverted settings listed by:
$ git grep MNT_INVERT libmount/src/optmap.c
This makes it clear that ‘defaults’ doesn't *change* anything or
return you to the default values of some subset of settings.
b. ‘defaults’ is supposed to mean “zero mountflags” (and maybe also
“zero data”?). In this case, like case a, we should drop it from
examples that set any other options (like [5]). We should update
the man entries [1,2] to explain the clearing. But I don't know
why anyone would ever use this option except on its own as a
fourth-fstab-field placeholder (like case a). Presumably whoever
is inserting ‘defaults’ in the mount options could just remove all
the earlier entries (e.g. using "bar" instead of
"foo,defaults,bar").
c. ‘defaults’ is supposed to be shorthand for
‘rw,suid,dev,exec,auto,nouser,async’. It should be implemented
with something like:
{ "defaults", MS_RDONLY | MS_NOSUID | MS_NODEV | MS_NOEXEC | MNT_MS_NOAUTO | MNT_MS_USER | MS_SYNCHRONOUS, MNT_NOHLPS | MNT_INVERT | MNT_NOMTAB},
(although I have no idea if using MS_RDONLY flags in
userspace_opts_map is viable. It probably isn't).
Altenatively, it could be implemented by removing the entry from
userspace_opts_map and adding a pre-processing step that replaces
shorthand (e.g. mapping "foo,defaults,bar" →
"foo,rw,suid,dev,exec,auto,nouser,async,bar") before iterating over
the options in mnt_context_merge_mflags. The man entries [1,2] are
fine.
d. I'm missing something obvious ;). We may want some really small
words in mount(8) to keep others from falling into the same
misunderstanding that I've fallen into.
Regardless of which case is correct, we may want to annotate the
inverted flags (as I mentioned in case a), since that (combined with
the existing default caveats in [6]) explains the “we will call
something close to mount(source, target, filesystemtype, 0, NULL)”
starting point, and folks might not think “let me look up the
‘defaults’ docs…” when they're trying to figure out if the default is
dev or nodev, etc.
Thoughts?
Trevor
[1]: https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/tree/sys-utils/mount.8?h=v2.29.2#n935
[2]: https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/tree/sys-utils/fstab.5?h=v2.29.2#n172
[3]: https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/tree/libmount/src/optmap.c?h=v2.29.2#n138
[4]: https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/tree/libmount/src/libmount.h.in?h=v2.29.2#n58
[5]: https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/tree/sys-utils/fstab.5?h=v2.29.2#n65
[6]: https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/tree/sys-utils/mount.8?h=v2.29.2#n824
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