implement "mount -o remount" support #4091
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Function teardown assumes that every test case will call setup_sshfs. Currently, this assumption is true, but once a test case that won't call setup_sshfs is added (say because it has some extra "requires" or "skip"), it will break bats, so any bats invocation involving such a test case will end up hanging after the last test case is run. The reason is, we have set -u in helpers.bash to help catching the use of undefined variables. In the above scenario, such a variable is DIR, which is referenced in teardown but is only defined after a call to setup_sshfs. As a result, bash that is running the teardown function will exit upon seeing the first $DIR, and thus teardown_bundle won't be run. This, in turn, results in a stale recvtty process, which inherits bats' fd 3. Until that fd is closed, bats waits for test logs. Long story short, the fix is to - check if DIR is set before referencing it; - unset it after unmount. PS it is still not clear why there is no diagnostics about the failed teardown. Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
The original reasoning for this option was to avoid having mount options be overwritten by runc. However, adding command-line arguments has historically been a bad idea because it forces strict-runc-compatible OCI runtimes to copy out-of-spec features directly from runc and these flags are usually quite difficult to enable by users when using runc through several layers of engines and orchestrators. A far more preferable solution is to have a heuristic which detects whether copying the original mount's mount options would override an explicit mount option specified by the user. In this case, we should return an error. You only end up in this path in the userns case, if you have a bind-mount source with locked flags. During the course of writing this patch, I discovered that several aspects of our handling of flags for bind-mounts left much to be desired. We have completely botched the handling of explicitly cleared flags since commit 97f5ee4 ("Only remount if requested flags differ from current"), with our behaviour only becoming increasingly more weird with 50105de ("Fix failure with rw bind mount of a ro fuse") and da780e4 ("Fix bind mounts of filesystems with certain options set"). In short, we would only clear flags explicitly request by the user purely by chance, in ways that it really should've been reported to us by now. The most egregious is that mounts explicitly marked "rw" were actually mounted "ro" if the bind-mount source was "ro" and no other special flags were included. In addition, our handling of atime was completely broken -- mostly due to how subtle the semantics of atime are on Linux. Unfortunately, while the runtime-spec requires us to implement mount(8)'s behaviour, several aspects of the util-linux mount(8)'s behaviour are broken and thus copying them makes little sense. Since the runtime-spec behaviour for this case (should mount options for a "bind" mount use the "mount --bind -o ..." or "mount --bind -o remount,..." semantics? Is the fallback code we have for userns actually spec-compliant?) and the mount(8) behaviour (see [1]) are not well-defined, this commit simply fixes the most obvious aspects of the behaviour that are broken while keeping the current spirit of the implementation. NOTE: The handling of atime in the base case is left for a future PR to deal with. This means that the atime of the source mount will be silently left alone unless the fallback path needs to be taken, and any flags not explicitly set will be cleared in the base case. Whether we should always be operating as "mount --bind -o remount,..." (where we default to the original mount source flags) is a topic for a separate PR and (probably) associated runtime-spec PR. So, to resolve this: * We store which flags were explicitly requested to be cleared by the user, so that we can detect whether the userns fallback path would end up setting a flag the user explicitly wished to clear. If so, we return an error because we couldn't fulfil the configuration settings. * Revert 97f5ee4 ("Only remount if requested flags differ from current"), as missing flags do not mean we can skip MS_REMOUNT (in fact, missing flags are how you indicate a flag needs to be cleared with mount(2)). The original purpose of the patch was to fix the userns issue, but as mentioned above the correct mechanism is to do a fallback mount that copies the lockable flags from statfs(2). * Improve handling of atime in the fallback case by: - Correctly handling the returned flags in statfs(2). - Implement the MNT_LOCK_ATIME checks in our code to ensure we produce errors rather than silently producing incorrect atime mounts. * Improve the tests so we correctly detect all of these contingencies, including a general "bind-mount atime handling" test to ensure that the behaviour described here is accurate. This change also inlines the remount() function -- it was only ever used for the bind-mount remount case, and its behaviour is very bind-mount specific. [1]: util-linux/util-linux#2433 Reverts: 97f5ee4 ("Only remount if requested flags differ from current") Fixes: 50105de ("Fix failure with rw bind mount of a ro fuse") Fixes: da780e4 ("Fix bind mounts of filesystems with certain options set") Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
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Having thought about it some more, I'm not sure I like this approach... Having a separate mount entry is arguably closer to |
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Split off from #3967.
We have a
remountflag in the runtime-spec, but its behaviour was not well-defined.Signed-off-by: Aleksa Sarai cyphar@cyphar.com