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This can happen when small amount of memory is allocated for tld_data_u. The misalignment is supposed to be allowed and the BPF side will use tld_data_u->start to reference the tld_data_u in a page. However, since "start" is within tld_data_u, there is no way to know the correct "start" in the first place. As a result, BPF programs will see garbage data. The selftest did not catch this since it tries to allocate the maximum amount of data possible (i.e., a page) such that tld_data_u->start is always correct. Fix it by moving tld_data_u->start to tld_data_map->start. The original field is now renamed as unused instead of removing it because BPF side tld_get_data() views off = 0 returned from tld_fetch_key() as uninitialized. Signed-off-by: Amery Hung --- .../testing/selftests/bpf/prog_tests/task_local_data.h | 10 ++++++++-- .../testing/selftests/bpf/progs/task_local_data.bpf.h | 5 +++-- 2 files changed, 11 insertions(+), 4 deletions(-) diff --git a/tools/testing/selftests/bpf/prog_tests/task_local_data.h b/tools/testing/selftests/bpf/prog_tests/task_local_data.h index 489f07045c9f..8ae4fb2027f7 100644 --- a/tools/testing/selftests/bpf/prog_tests/task_local_data.h +++ b/tools/testing/selftests/bpf/prog_tests/task_local_data.h @@ -99,14 +99,20 @@ struct tld_meta_u { struct tld_metadata metadata[]; }; +/* + * The unused field ensures map_val.start > 0. On the BPF side, __tld_fetch_key() + * calculates off by summing map_val.start and tld_key_t.off and treats off == 0 + * as key not cached. + */ struct tld_data_u { - __u64 start; /* offset of tld_data_u->data in a page */ + __u64 unused; char data[] __attribute__((aligned(8))); }; struct tld_map_value { void *data; struct tld_meta_u *meta; + __u16 start; /* offset of tld_data_u->data in a page */ }; struct tld_meta_u * _Atomic tld_meta_p __attribute__((weak)); @@ -182,7 +188,7 @@ static int __tld_init_data_p(int map_fd) * is a page in BTF. */ map_val.data = (void *)(TLD_PAGE_MASK & (intptr_t)data); - data->start = (~TLD_PAGE_MASK & (intptr_t)data) + sizeof(struct tld_data_u); + map_val.start = (~TLD_PAGE_MASK & (intptr_t)data) + sizeof(struct tld_data_u); map_val.meta = tld_meta_p; err = bpf_map_update_elem(map_fd, &tid_fd, &map_val, 0); diff --git a/tools/testing/selftests/bpf/progs/task_local_data.bpf.h b/tools/testing/selftests/bpf/progs/task_local_data.bpf.h index 1f396711f487..0df8a12fd61e 100644 --- a/tools/testing/selftests/bpf/progs/task_local_data.bpf.h +++ b/tools/testing/selftests/bpf/progs/task_local_data.bpf.h @@ -86,13 +86,14 @@ struct tld_meta_u { }; struct tld_data_u { - __u64 start; /* offset of tld_data_u->data in a page */ + __u64 unused; char data[__PAGE_SIZE - sizeof(__u64)] __attribute__((aligned(8))); }; struct tld_map_value { struct tld_data_u __uptr *data; struct tld_meta_u __uptr *meta; + __u16 start; /* offset of tld_data_u->data in a page */ }; typedef struct tld_uptr_dummy { @@ -176,7 +177,7 @@ static int __tld_fetch_key(struct tld_object *tld_obj, const char *name, int i_s if (!tld_obj->data_map || !tld_obj->data_map->data || !tld_obj->data_map->meta) return 0; - start = tld_obj->data_map->data->start; + start = tld_obj->data_map->start; cnt = tld_obj->data_map->meta->cnt; metadata = tld_obj->data_map->meta->metadata; -- 2.52.0