* [bug report] btrfs: fix corrupt read due to bad offset of a compressed extent map
@ 2026-06-15 12:46 Dan Carpenter
2026-06-15 12:56 ` Filipe Manana
0 siblings, 1 reply; 3+ messages in thread
From: Dan Carpenter @ 2026-06-15 12:46 UTC (permalink / raw)
To: Filipe Manana; +Cc: linux-btrfs
Hello Filipe Manana,
Commit de9f46cb0044 ("btrfs: fix corrupt read due to bad offset of a
compressed extent map") from Jul 11, 2024 (linux-next), leads to the
following Smatch static checker warning:
fs/btrfs/tests/extent-map-tests.c:979 test_case_8()
error: dereferencing freed memory 'em' (line 973 btrfs_free_extent_map())
fs/btrfs/tests/extent-map-tests.c
912 static int test_case_8(struct btrfs_fs_info *fs_info, struct btrfs_inode *inode)
913 {
914 struct extent_map_tree *em_tree = &inode->extent_tree;
915 struct extent_map *em;
916 int ret;
917 int ret2;
918
919 em = btrfs_alloc_extent_map();
920 if (!em) {
921 test_std_err(TEST_ALLOC_EXTENT_MAP);
922 return -ENOMEM;
923 }
924
925 /* Compressed extent for the file range [120K, 128K). */
926 em->start = SZ_1K * 120;
927 em->len = SZ_8K;
928 em->disk_num_bytes = SZ_4K;
929 em->ram_bytes = SZ_8K;
930 em->flags |= EXTENT_FLAG_COMPRESS_ZLIB;
931 write_lock(&em_tree->lock);
932 ret = btrfs_add_extent_mapping(inode, &em, em->start, em->len);
933 write_unlock(&em_tree->lock);
934 btrfs_free_extent_map(em);
935 if (ret < 0) {
936 test_err("couldn't add extent map for range [120K, 128K)");
937 goto out;
938 }
939
940 em = btrfs_alloc_extent_map();
941 if (!em) {
942 test_std_err(TEST_ALLOC_EXTENT_MAP);
943 ret = -ENOMEM;
944 goto out;
945 }
946
947 /*
948 * Compressed extent for the file range [108K, 144K), which overlaps
949 * with the [120K, 128K) we previously inserted.
950 */
951 em->start = SZ_1K * 108;
952 em->len = SZ_1K * 36;
953 em->disk_num_bytes = SZ_4K;
954 em->ram_bytes = SZ_1K * 36;
955 em->flags |= EXTENT_FLAG_COMPRESS_ZLIB;
956
957 /*
958 * Try to add the extent map but with a search range of [140K, 144K),
959 * this should succeed and adjust the extent map to the range
960 * [128K, 144K), with a length of 16K and an offset of 20K.
961 *
962 * This simulates a scenario where in the subvolume tree of an inode we
963 * have a compressed file extent item for the range [108K, 144K) and we
964 * have an overlapping compressed extent map for the range [120K, 128K),
965 * which was created by an encoded write, but its ordered extent was not
966 * yet completed, so the subvolume tree doesn't have yet the file extent
967 * item for that range - we only have the extent map in the inode's
968 * extent map tree.
969 */
970 write_lock(&em_tree->lock);
971 ret = btrfs_add_extent_mapping(inode, &em, SZ_1K * 140, SZ_4K);
972 write_unlock(&em_tree->lock);
973 btrfs_free_extent_map(em);
This looks like btrfs_free_extent_map() frees "em".
974 if (ret < 0) {
975 test_err("couldn't add extent map for range [108K, 144K)");
976 goto out;
977 }
978
--> 979 if (em->start != SZ_128K) {
^^^^^^^^^
980 test_err("unexpected extent map start %llu (should be 128K)", em->start);
^^^^^^^^^
981 ret = -EINVAL;
982 goto out;
983 }
984 if (em->len != SZ_16K) {
^^^^^^^
985 test_err("unexpected extent map length %llu (should be 16K)", em->len);
^^^^^^^
986 ret = -EINVAL;
987 goto out;
988 }
989 if (em->offset != SZ_1K * 20) {
^^^^^^^^^^
990 test_err("unexpected extent map offset %llu (should be 20K)", em->offset);
^^^^^^^^^^
991 ret = -EINVAL;
992 goto out;
993 }
994 out:
995 ret2 = free_extent_map_tree(inode);
996 if (ret == 0)
997 ret = ret2;
998
999 return ret;
1000 }
This email is a free service from the Smatch-CI project [smatch.sf.net].
regards,
dan carpenter
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [bug report] btrfs: fix corrupt read due to bad offset of a compressed extent map
2026-06-15 12:46 [bug report] btrfs: fix corrupt read due to bad offset of a compressed extent map Dan Carpenter
@ 2026-06-15 12:56 ` Filipe Manana
2026-06-15 13:12 ` Dan Carpenter
0 siblings, 1 reply; 3+ messages in thread
From: Filipe Manana @ 2026-06-15 12:56 UTC (permalink / raw)
To: Dan Carpenter; +Cc: Filipe Manana, linux-btrfs
On Mon, Jun 15, 2026 at 1:47 PM Dan Carpenter <error27@gmail.com> wrote:
>
> Hello Filipe Manana,
>
> Commit de9f46cb0044 ("btrfs: fix corrupt read due to bad offset of a
> compressed extent map") from Jul 11, 2024 (linux-next), leads to the
> following Smatch static checker warning:
>
> fs/btrfs/tests/extent-map-tests.c:979 test_case_8()
> error: dereferencing freed memory 'em' (line 973 btrfs_free_extent_map())
>
> fs/btrfs/tests/extent-map-tests.c
> 912 static int test_case_8(struct btrfs_fs_info *fs_info, struct btrfs_inode *inode)
> 913 {
> 914 struct extent_map_tree *em_tree = &inode->extent_tree;
> 915 struct extent_map *em;
> 916 int ret;
> 917 int ret2;
> 918
> 919 em = btrfs_alloc_extent_map();
> 920 if (!em) {
> 921 test_std_err(TEST_ALLOC_EXTENT_MAP);
> 922 return -ENOMEM;
> 923 }
> 924
> 925 /* Compressed extent for the file range [120K, 128K). */
> 926 em->start = SZ_1K * 120;
> 927 em->len = SZ_8K;
> 928 em->disk_num_bytes = SZ_4K;
> 929 em->ram_bytes = SZ_8K;
> 930 em->flags |= EXTENT_FLAG_COMPRESS_ZLIB;
> 931 write_lock(&em_tree->lock);
> 932 ret = btrfs_add_extent_mapping(inode, &em, em->start, em->len);
> 933 write_unlock(&em_tree->lock);
> 934 btrfs_free_extent_map(em);
> 935 if (ret < 0) {
> 936 test_err("couldn't add extent map for range [120K, 128K)");
> 937 goto out;
> 938 }
> 939
> 940 em = btrfs_alloc_extent_map();
> 941 if (!em) {
> 942 test_std_err(TEST_ALLOC_EXTENT_MAP);
> 943 ret = -ENOMEM;
> 944 goto out;
> 945 }
> 946
> 947 /*
> 948 * Compressed extent for the file range [108K, 144K), which overlaps
> 949 * with the [120K, 128K) we previously inserted.
> 950 */
> 951 em->start = SZ_1K * 108;
> 952 em->len = SZ_1K * 36;
> 953 em->disk_num_bytes = SZ_4K;
> 954 em->ram_bytes = SZ_1K * 36;
> 955 em->flags |= EXTENT_FLAG_COMPRESS_ZLIB;
> 956
> 957 /*
> 958 * Try to add the extent map but with a search range of [140K, 144K),
> 959 * this should succeed and adjust the extent map to the range
> 960 * [128K, 144K), with a length of 16K and an offset of 20K.
> 961 *
> 962 * This simulates a scenario where in the subvolume tree of an inode we
> 963 * have a compressed file extent item for the range [108K, 144K) and we
> 964 * have an overlapping compressed extent map for the range [120K, 128K),
> 965 * which was created by an encoded write, but its ordered extent was not
> 966 * yet completed, so the subvolume tree doesn't have yet the file extent
> 967 * item for that range - we only have the extent map in the inode's
> 968 * extent map tree.
> 969 */
> 970 write_lock(&em_tree->lock);
> 971 ret = btrfs_add_extent_mapping(inode, &em, SZ_1K * 140, SZ_4K);
> 972 write_unlock(&em_tree->lock);
> 973 btrfs_free_extent_map(em);
>
> This looks like btrfs_free_extent_map() frees "em".
Nop, false alarm.
And that's because btrfs_add_extent_mapping() will increase the ref
count of the extent map if it returns success (one ref for the tree).
So until the extent map is removed from the tree, we can use the em
after calling btrfs_free_extent_map().
Thanks.
>
> 974 if (ret < 0) {
> 975 test_err("couldn't add extent map for range [108K, 144K)");
> 976 goto out;
> 977 }
> 978
> --> 979 if (em->start != SZ_128K) {
> ^^^^^^^^^
>
> 980 test_err("unexpected extent map start %llu (should be 128K)", em->start);
> ^^^^^^^^^
> 981 ret = -EINVAL;
> 982 goto out;
> 983 }
> 984 if (em->len != SZ_16K) {
> ^^^^^^^
>
> 985 test_err("unexpected extent map length %llu (should be 16K)", em->len);
> ^^^^^^^
> 986 ret = -EINVAL;
> 987 goto out;
> 988 }
> 989 if (em->offset != SZ_1K * 20) {
> ^^^^^^^^^^
> 990 test_err("unexpected extent map offset %llu (should be 20K)", em->offset);
> ^^^^^^^^^^
> 991 ret = -EINVAL;
> 992 goto out;
> 993 }
> 994 out:
> 995 ret2 = free_extent_map_tree(inode);
> 996 if (ret == 0)
> 997 ret = ret2;
> 998
> 999 return ret;
> 1000 }
>
> This email is a free service from the Smatch-CI project [smatch.sf.net].
>
> regards,
> dan carpenter
>
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [bug report] btrfs: fix corrupt read due to bad offset of a compressed extent map
2026-06-15 12:56 ` Filipe Manana
@ 2026-06-15 13:12 ` Dan Carpenter
0 siblings, 0 replies; 3+ messages in thread
From: Dan Carpenter @ 2026-06-15 13:12 UTC (permalink / raw)
To: Filipe Manana; +Cc: Filipe Manana, linux-btrfs
On Mon, Jun 15, 2026 at 01:56:22PM +0100, Filipe Manana wrote:
> On Mon, Jun 15, 2026 at 1:47 PM Dan Carpenter <error27@gmail.com> wrote:
> > 957 /*
> > 958 * Try to add the extent map but with a search range of [140K, 144K),
> > 959 * this should succeed and adjust the extent map to the range
> > 960 * [128K, 144K), with a length of 16K and an offset of 20K.
> > 961 *
> > 962 * This simulates a scenario where in the subvolume tree of an inode we
> > 963 * have a compressed file extent item for the range [108K, 144K) and we
> > 964 * have an overlapping compressed extent map for the range [120K, 128K),
> > 965 * which was created by an encoded write, but its ordered extent was not
> > 966 * yet completed, so the subvolume tree doesn't have yet the file extent
> > 967 * item for that range - we only have the extent map in the inode's
> > 968 * extent map tree.
> > 969 */
> > 970 write_lock(&em_tree->lock);
> > 971 ret = btrfs_add_extent_mapping(inode, &em, SZ_1K * 140, SZ_4K);
> > 972 write_unlock(&em_tree->lock);
> > 973 btrfs_free_extent_map(em);
> >
> > This looks like btrfs_free_extent_map() frees "em".
>
> Nop, false alarm.
> And that's because btrfs_add_extent_mapping() will increase the ref
> count of the extent map if it returns success (one ref for the tree).
> So until the extent map is removed from the tree, we can use the em
> after calling btrfs_free_extent_map().
>
Ah. Thanks for taking a look...
regards,
dan carpenter
^ permalink raw reply [flat|nested] 3+ messages in thread
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2026-06-15 12:46 [bug report] btrfs: fix corrupt read due to bad offset of a compressed extent map Dan Carpenter
2026-06-15 12:56 ` Filipe Manana
2026-06-15 13:12 ` Dan Carpenter
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