According to the Al Viro[1], kernel has no way to detect that a single file is used by multiple loop devices, and multiple mounts of the same file using different loop devices will result in a data corruption. Exactly this now happens, if multiple btrfs sub-volumes in one file are mounted with "-oloop". Make use of multiple -oloop mounting the same file safe: Do a loop devices lookup, and if a loop device is already initialized, use it. Hopefully it is possible, as "losetup -d" will return OK, even if the device itself is in use, and is not released. Problems: There is a risk of race condition between the lookup and real mount. Once loop device is initialized read-only, kernel offers no way to turn it to read-write. It has to fail. References: https://lkml.org/lkml/2016/2/26/897 Signed-off-by: Stanislav Brabec <sbrabec@xxxxxxx> --- libmount/src/context_loopdev.c | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/libmount/src/context_loopdev.c b/libmount/src/context_loopdev.c index 47d6294..0aa91ea 100644 --- a/libmount/src/context_loopdev.c +++ b/libmount/src/context_loopdev.c @@ -210,6 +210,29 @@ int mnt_context_setup_loopdev(struct libmnt_context *cxt) if (rc) goto done_no_deinit; + /* It is possible to mount the same file more times. If we set more + * than one loop device referring to the same file, kernel has no + * mechanism to detect it. To prevent data corruption, the same loop + * device has to be recycled. + */ + rc = loopcxt_init(&lc, 0); + if (rc) + goto done; + if (backing_file && !(loopcxt_find_by_backing_file(&lc, + backing_file, offset, LOOPDEV_FL_OFFSET))) { + DBG(LOOP, ul_debugobj(cxt, "using existing loop device %s", + loopcxt_get_device(&lc))); + /* Once a loop is initialized RO, there is no way to safely + mount that file in R/W mode. */ + if (loopcxt_is_readonly(&lc) && !(lo_flags & LO_FLAGS_READ_ONLY)) { + rc = -EROFS; + goto done; + } + + goto success; + } + loopcxt_deinit(&lc); + rc = loopcxt_init(&lc, 0); if (rc == 0 && loopval) { rc = loopcxt_set_device(&lc, loopval); @@ -267,6 +290,7 @@ int mnt_context_setup_loopdev(struct libmnt_context *cxt) DBG(LOOP, ul_debugobj(cxt, "device stolen...trying again")); } while (1); +success: if (!rc) rc = mnt_fs_set_source(cxt->fs, loopcxt_get_device(&lc)); -- 2.8.1 -- Best Regards / S pozdravem, Stanislav Brabec software developer --------------------------------------------------------------------- SUSE LINUX, s. r. o. e-mail: sbrabec@xxxxxxxx Lihovarská 1060/12 tel: +49 911 7405384547 190 00 Praha 9 fax: +420 284 084 001 Czech Republic http://www.suse.cz/ PGP: 830B 40D5 9E05 35D8 5E27 6FA3 717C 209F A04F CD76 -- To unsubscribe from this list: send the line "unsubscribe util-linux" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
