Bug#1121013: linux: mksquashfs segfaults ~20% of the times with kernel 6.18
Hi,
On Wed, Nov 19, 2025 at 03:03:51PM +0000, Luca Boccassi wrote:
> Source: linux
> Version: 6.18~rc6-1~exp1
> Severity: serious
> Justification: breaks other package's autopkgtest
>
> With kernel 6.18 from experimental mksquashfs segfaults roughly 1 in 4
> invocations. This does not happen with the kernel in unstable/testing,
> so it looks like a kernel regression.
>
> Filing at serious as it breaks systemd's autopkgtest:
> https://ci.debian.net/packages/s/systemd/unstable/amd64/66358275/#S67
>
> Trivial to reproduce:
>
> mkdir -p bar
> while mksquashfs bar bar.raw -noappend &>/dev/null; do true; done
>
> Decoded backtrace is strange, it looks like a pointer is corrupted.
> Different invocations result in slightly different crashes, although
> all seem to be in the xattr code handling, so that looks like a strong
> hint as to where things might have regressed.
>
> https://sources.debian.org/src/squashfs-tools/1%3A4.7.4-1/squashfs-tools/xattr.c#L631
>
> #0 0x000055e3c9fddcd9 in read_xattrs (d=d@entry=0x55e3d1388be0,
> type=type@entry=1) at ./squashfs-tools/xattr.c:631
> entry = 0x40e33
> dir_ent = <optimized out>
> inode = <optimized out>
> filename = 0x7ffeb945bdbb "bar"
> xattr_list = 0x0
> head = 0x0
> count = 0
> i = <optimized out>
> j = <optimized out>
> l1 = <error reading variable l1 (Cannot access memory at
> address 0x40e4b)>
> l2 = <optimized out>
> l3 = <optimized out>
> action_add_list = 0x0
> __func__ = "read_xattrs"
> #1 0x000055e3c9fb571f in create_inode
> (dir_info=dir_info@entry=0x55e3d1388b70, dir_ent=0x55e3d1388be0,
> type=type@entry=1, byte_size=byte_size@entry=3,
> start_block=start_block@entry=0, offset=offset@entry=0,
> block_list=0x0, fragment=0x0, dir_in=0x7ffeb9459840, sparse=0) at
> ./squashfs-tools/mksquashfs.c:1112
> buf = 0x55e3d1388c30
> inode_header = {base = {inode_type = 0, mode = 0, uid = 0,
> guid = 0, mtime = 3599334970,
> inode_number = 32632}, dev = {inode_type = 0, mode = 0,
> uid = 0, guid = 0, mtime = 3599334970,
> inode_number = 32632, nlink = 0, rdev = 0}, ldev =
> {inode_type = 0, mode = 0, uid = 0, guid = 0,
> mtime = 3599334970, inode_number = 32632, nlink = 0, rdev
> = 0, xattr = 24080}, symlink = {
> inode_type = 0, mode = 0, uid = 0, guid = 0, mtime =
> 3599334970, inode_number = 32632, nlink = 0,
> symlink_size = 0, symlink = 0x7ffeb9459748 "\020^"}, reg =
> {inode_type = 0, mode = 0, uid = 0, guid = 0,
> mtime = 3599334970, inode_number = 32632, start_block = 0,
> fragment = 0, offset = 24080, file_size = 0,
> block_list = 0x7ffeb9459750}, lreg = {inode_type = 0, mode
> = 0, uid = 0, guid = 0, mtime = 3599334970,
> inode_number = 32632, start_block = 0, file_size = 24080,
> sparse = 0, nlink = 0, fragment = 0,
> offset = 0, xattr = 0, block_list = 0x7ffeb9459768}, dir =
> {inode_type = 0, mode = 0, uid = 0, guid = 0,
> mtime = 3599334970, inode_number = 32632, start_block = 0,
> nlink = 0, file_size = 24080, offset = 0,
> parent_inode = 0}, ldir = {inode_type = 0, mode = 0, uid =
> 0, guid = 0, mtime = 3599334970,
> inode_number = 32632, nlink = 0, file_size = 0,
> start_block = 24080, parent_inode = 0, i_count = 0,
> offset = 0, xattr = 0, index = 0x7ffeb9459758}, ipc =
> {inode_type = 0, mode = 0, uid = 0, guid = 0,
> mtime = 3599334970, inode_number = 32632, nlink = 0}, lipc
> = {inode_type = 0, mode = 0, uid = 0,
> guid = 0, mtime = 3599334970, inode_number = 32632, nlink
> = 0, xattr = 0}}
> base = 0x7ffeb9459730
> inode = <optimized out>
> filename = 0x7ffeb945bdbb "bar"
> nlink = 1
> xattr = <optimized out>
> uid = <optimized out>
> gid = <optimized out>
> mode = <optimized out>
> #2 0x000055e3c9fb68a0 in write_dir (dir_info=<optimized out>,
> dir=0x7ffeb9459840)
> at ./squashfs-tools/mksquashfs.c:1522
> dir_size = <optimized out>
> data_space = <optimized out>
> directory_block = <optimized out>
> directory_offset = <optimized out>
> i_count = 0
> index = 16384
> c_byte = <optimized out>
> cache = <optimized out>
> __func__ = "write_dir"
> #3 dir_scan8 (inode=<optimized out>, dir_info=<optimized out>) at
> ./squashfs-tools/mksquashfs.c:4647
> squashfs_type = <optimized out>
> dir = <optimized out>
> dir_ent = <optimized out>
> file = <optimized out>
> #4 0x000055e3c9fbaa85 in do_directory_scans
> (dir_ent=dir_ent@entry=0x55e3d1388be0, progress=progress@entry=1)
> at ./squashfs-tools/mksquashfs.c:3620
> inode = 208
> pseudo = <optimized out>
> #5 0x000055e3c9fbc041 in scan_single (pathname=0x7ffeb945bdbb "bar",
> progress=progress@entry=1)
> at ./squashfs-tools/mksquashfs.c:3675
> buf = {st_dev = 32, st_ino = 21, st_nlink = 2, st_mode =
> 16877, st_uid = 0, st_gid = 0, __pad0 = 0,
> st_rdev = 0, st_size = 40, st_blksize = 4096, st_blocks = 0,
> st_atim = {tv_sec = 1763563405,
> tv_nsec = 364000000}, st_mtim = {tv_sec = 1763562938,
> tv_nsec = 96000000}, st_ctim = {
> tv_sec = 1763562938, tv_nsec = 96000000}, __glibc_reserved
> = {0, 0, 0}}
> dir_ent = 0x55e3d1388be0
> #6 0x000055e3c9fac6b7 in dir_scan (directory=<optimized out>,
> progress=1) at ./squashfs-tools/mksquashfs.c:3735
> single = <optimized out>
> #7 main (argc=<optimized out>, argv=<optimized out>) at
> ./squashfs-tools/mksquashfs.c:8769
> buf = {st_dev = 32, st_ino = 22, st_nlink = 1, st_mode =
> 33188, st_uid = 0, st_gid = 0, __pad0 = 0,
> st_rdev = 0, st_size = 4096, st_blksize = 4096, st_blocks =
> 8, st_atim = {tv_sec = 1763562951,
> tv_nsec = 448000000}, st_mtim = {tv_sec = 1763563405,
> tv_nsec = 360000000}, st_ctim = {
> tv_sec = 1763563405, tv_nsec = 360000000},
> __glibc_reserved = {0, 0, 0}}
> source_buf = {st_dev = 32, st_ino = 21, st_nlink = 2, st_mode
> = 16877, st_uid = 0, st_gid = 0, __pad0 = 0,
> st_rdev = 0, st_size = 40, st_blksize = 4096, st_blocks = 0,
> st_atim = {tv_sec = 1763563405,
> tv_nsec = 360000000}, st_mtim = {tv_sec = 1763562938,
> tv_nsec = 96000000}, st_ctim = {
> tv_sec = 1763562938, tv_nsec = 96000000}, __glibc_reserved
> = {0, 0, 0}}
> res = 0
> i = <optimized out>
> j = <optimized out>
> root_name = <optimized out>
> inode = <optimized out>
> readq = 496
> fragq = 498
> bwriteq = 496
> fwriteq = <optimized out>
> total_mem = <optimized out>
> progress = 1
> force_progress = <optimized out>
> percentage = <optimized out>
> exclude_option = 0
> Xhelp = <optimized out>
> fragment = 0x0
> command = <optimized out>
> single_threaded = <optimized out>
> overcommit = 0
> repro_opt = <optimized out>
> repro_time_opt = <optimized out>
> repro_time = 4
> __func__ = "main"
> (gdb) p l1
> Cannot access memory at address 0x40e4b
> (gdb) p xattr_add_list
> $1 = (struct xattr_add *) 0x0
>
> https://sources.debian.org/src/squashfs-tools/1%3A4.7.4-1/squashfs-tools/xattr.c#L534
>
> #0 0x000055a5314fb9e0 in sort_list (head=head@entry=0x55a531fcfa50
> <xattr_add_list>, count=54720)
> at ./squashfs-tools/xattr.c:534
> cur = <optimized out>
> l1 = <optimized out>
> l2 = 0x83500e000000005d
> next = <optimized out>
> len1 = 0
> len2 = <optimized out>
> stride = 1
> #1 0x000055a5314fda75 in sort_list (head=0x55a531fcfa50
> <xattr_add_list>, count=<optimized out>)
> at ./squashfs-tools/xattr.c:534
> cur = <optimized out>
> l1 = <optimized out>
> l2 = <optimized out>
> next = <optimized out>
> len1 = <optimized out>
> len2 = <optimized out>
> stride = 1
> #2 0x000055a5314ca2cf in main (argc=<optimized out>,
> argv=0x7fff771b2b58) at ./squashfs-tools/mksquashfs.c:8381
> buf = {st_dev = 60405, st_ino = 4096, st_nlink = 8192, st_mode
> = 5, st_uid = 0, st_gid = 61440, __pad0 = 0,
> st_rdev = 69632, st_size = 67156, st_blksize = 67156,
> st_blocks = 4096, st_atim = {tv_sec = 61440,
> tv_nsec = 1}, st_mtim = {tv_sec = 69632, tv_nsec = 77824},
> st_ctim = {tv_sec = 73736, tv_nsec = 73856},
> __glibc_reserved = {4096, 65536, 3}}
> source_buf = {st_dev = 4, st_ino = 17179869188, st_nlink =
> 1975252, st_mode = 1975252, st_uid = 0,
> st_gid = 1975252, __pad0 = 0, st_rdev = 32, st_size = 32,
> st_blksize = 4, st_blocks = 17179869191,
> st_atim = {tv_sec = 1977176, tv_nsec = 1981272}, st_mtim =
> {tv_sec = 1981272, tv_nsec = 16}, st_ctim = {
> tv_sec = 136, tv_nsec = 8}, __glibc_reserved =
> {18865251667, 904, 904}}
> res = 0
> i = 4
> j = <optimized out>
> root_name = <optimized out>
> inode = <optimized out>
> readq = 496
> fragq = 498
> bwriteq = 496
> fwriteq = <optimized out>
> total_mem = <optimized out>
> progress = 1
> force_progress = <optimized out>
> percentage = <optimized out>
> exclude_option = 0
> Xhelp = <optimized out>
> fragment = 0x0
> command = <optimized out>
> single_threaded = <optimized out>
> overcommit = 0
> repro_opt = <optimized out>
> repro_time_opt = <optimized out>
> repro_time = 4
> __func__ = "main"
This seems to regress from v6.18-rc5 to v6.18-rc6 so let's see what
bisecting the upstream version shows now.
Regards,
Salvatore
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