[LTP] [PATCH v2 3/3] tst_find_backing_dev(): add support for overlayfs

[Petr Vorel]

Hi Jeff,

> Add checks for overlayfs in tst_find_backing_dev.  As with btrfs, only
> a single device is checked (the upper one) and returned from
> tst_find_backing_dev().

> The implementation uses both /proc/self/mountinfo and /proc/self/mounts.
> The former is used to map a device to a mountpoint, and the latter is
> used to get the file system options for the mountpoint.  All of the
> information is present in mountinfo, but the file format is more complex,
> and there are no glibc helpers for parsing it.

> The '#define _GNU_SOURCE' was added for the use of the strchrnul(3)
> function.

+1 (strangely not needed for current glibc in Tumbleweed, but at least musl
requires it).

> Signed-off-by: Jeff Moyer <jmoyer@redhat.com>

> ---
> v2: Don't use libmount.  Instead, map from device number to mount-point
>     using /proc/self/mountinfo, and then use the mntent.h helpers to get
>     the mount options for the mountpoint from /proc/self/mounts.

Thanks a lot!

> ---
>  lib/tst_device.c | 134 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 134 insertions(+)

> diff --git a/lib/tst_device.c b/lib/tst_device.c
> index 744e08a68..4730396b4 100644
> --- a/lib/tst_device.c
> +++ b/lib/tst_device.c
> @@ -3,6 +3,7 @@
>   * Copyright (C) 2014 Cyril Hrubis chrubis@suse.cz
>   */

> +#define _GNU_SOURCE
>  #include <sys/types.h>
>  #include <sys/stat.h>
>  #include <sys/ioctl.h>
> @@ -573,6 +574,137 @@ static void btrfs_get_uevent_path(char *tmp_path, char *uevent_path)
>  	SAFE_CLOSEDIR(NULL, dir);
>  }

> +static char *overlay_mount_from_dev(dev_t dev)
> +{
> +	unsigned dev_major, dev_minor, mnt_major, mnt_minor;
> +	FILE *fp;
> +	char line[PATH_MAX];
> +	char *mountpoint;
> +	int ret;
> +
> +	dev_major = major(dev);
> +	dev_minor = minor(dev);
> +
> +	fp = SAFE_FOPEN(NULL, "/proc/self/mountinfo", "r");
> +	while (fgets(line, sizeof(line), fp) != NULL) {
> +		ret = sscanf(line, "%*d %*d %u:%u %*s %ms",
> +			     &mnt_major, &mnt_minor, &mountpoint);
Can you please use SAFE_SSCANF() (our extra checks)?
> +		if (ret != 3)
> +			tst_brkm(TBROK, NULL,
> +				 "failed to parse mountinfo line: \"%s\"",
> +				 line);
> +		if (mnt_major == dev_major && mnt_minor == dev_minor)
> +			break;
> +		free(mountpoint);
> +		mountpoint = NULL;
> +	}
> +	SAFE_FCLOSE(NULL, fp);
> +	if (!mountpoint)
> +		tst_brkm(TBROK, NULL,
> +			 "Unable to find mount entry for device %u:%u\n",
> +			 dev_major, dev_minor);
> +
> +	return mountpoint;
> +}
> +
> +static char *overlay_get_upperdir(char *mountpoint)
> +{
> +	FILE *mntf;
> +	struct mntent *mnt;
> +	char *optstr, *optstart, *optend;
> +	char *upperdir = NULL;
> +
> +	mntf = setmntent("/proc/self/mounts", "r");
> +	if (!mntf)
> +		tst_brkm(TBROK | TERRNO, NULL, "Can't open /proc/self/mounts");
> +
> +	while ((mnt = getmntent(mntf)) != NULL) {
> +		if (strncmp(mnt->mnt_dir, mountpoint, strlen(mountpoint)))
> +			continue;
> +
> +		if (strncmp(mnt->mnt_type, "overlay", strlen("overlay")))
> +			tst_brkm(TBROK, NULL,
> +				 "expected overlay file system on mount point "\
> +				 "\"%s\", but it is of type %s.",
nit: I slightly prefer not splitting string when together is not that long.

> +				 mountpoint, mnt->mnt_type);
> +
> +		optstr = hasmntopt(mnt, "upperdir");
> +		if (optstr) {
> +			optstart = strchr(optstr, '=');
> +			optstart++;
> +			optend = strchrnul(optstr, ',');
> +			upperdir = calloc(optend - optstart + 1, 1);
> +			memcpy(upperdir, optstart, optend - optstart);
> +			break;
> +		} else {
> +			tst_brkm(TBROK, NULL,
> +				 "mount point %s does not contain an upperdir",
> +				 mountpoint);
> +		}
> +	}
> +	endmntent(mntf);
> +
> +	if (!upperdir)
> +		tst_brkm(TBROK, NULL,
> +			 "Unable to find mount point \"%s\" in mount table",
> +			 mountpoint);
> +
> +	return upperdir;
> +}
> +
> +/*
> + * To get from a file or directory on an overlayfs to a device
> + * for an underlying mountpoint requires the following steps:
> + *
> + * 1. stat() the pathname and pick out st_dev.
> + * 2. use the st_dev to look up the mount point of the file
> + *    system in /proc/self/mountinfo
> + *
> + * Because 'mountinfo' is a much more complicated file format than
> + * 'mounts', we switch to looking up the mount point in /proc/mounts,
> + * and use the mntent.h helpers to parse the entries.
+1

> + *
> + * 3. Using getmntent(), find the entry for the mount point identified
> + *    in step 2.
> + * 4. Call hasmntopt() to find the upperdir option, and parse that
> + *    option to get to the path name for the upper directory.
> + * 5. Call stat on the upper directory.  This should now contain
> + *    the major and minor number for the underlying device (assuming
> + *    that there aren't nested overlay file systems).
> + * 6. Populate the uevent path.
> + *
> + * Example /proc/self/mountinfo line for overlayfs:
> + *    471 69 0:48 / /tmp rw,relatime shared:242 - overlay overlay rw,seclabel,lowerdir=/tmp,upperdir=/mnt/upper/upper,workdir=/mnt/upper/work,uuid=null
> + *
> + * See section 3.5 of the kernel's Documentation/filesystems/proc.rst file
> + * for a detailed explanation of the mountinfo format.
+1, thanks for an example and docs.

> + */
> +static void overlay_get_uevent_path(char *tmp_path, char *uevent_path)
> +{
> +	int ret;
> +	struct stat st;
> +	char *mountpoint, *upperdir;
> +
> +	tst_resm(TINFO, "Use OVERLAYFS specific strategy");
> +
> +	ret = stat(tmp_path, &st);
> +	if (ret)
> +		tst_brkm(TBROK | TERRNO, NULL, "stat failed");
> +
> +	mountpoint = overlay_mount_from_dev(st.st_dev);
> +	upperdir = overlay_get_upperdir(mountpoint);
> +	free(mountpoint);
> +
> +	ret = stat(upperdir, &st);
> +	free(upperdir);
> +	if (ret)
> +		tst_brkm(TBROK | TERRNO, NULL, "stat failed");
> +
> +	tst_resm(TINFO, "Warning: used first of multiple backing devices.");
> +	sprintf(uevent_path, "/sys/dev/block/%d:%d/uevent",
> +		major(st.st_dev), minor(st.st_dev));
> +}
> +
>  __attribute__((nonnull))
>  void tst_find_backing_dev(const char *path, char *dev, size_t dev_size)
>  {
> @@ -600,6 +732,8 @@ void tst_find_backing_dev(const char *path, char *dev, size_t dev_size)

>  	if (fsbuf.f_type == BTRFS_SUPER_MAGIC) {
>  		btrfs_get_uevent_path(tmp_path, uevent_path);
> +	} else if (fsbuf.f_type == OVERLAYFS_SUPER_MAGIC) {
As I noted, we have ST_BTRFS_MAGIC and TST_OVERLAYFS_MAGIC in tst_fs.h.
And the reason is now clear, old headers we still support does not have it:

tst_device.c:735:29: error: 'OVERLAYFS_SUPER_MAGIC' undeclared (first use in this function)

https://github.com/pevik/ltp/actions/runs/13274657394/job/37061743549

With this fixed:
Reviewed-by: Petr Vorel <pvorel@suse.cz>

Kind regards,
Petr

> +		overlay_get_uevent_path(tmp_path, uevent_path);
>  	} else if (dev_major == 0) {
>  		tst_brkm(TBROK, NULL, "%s resides on an unsupported pseudo-file system.", path);
>  	} else {