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libbpf: Fix realloc API handling in zero-sized edge cases

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realloc() and reallocarray() can either return NULL or a special
non-NULL pointer, if their size argument is zero. This requires a bit
more care to handle NULL-as-valid-result situation differently from
NULL-as-error case. This has caused real issues before ([0]), and just
recently bit again in production when performing bpf_program__attach_usdt().

This patch fixes 4 places that do or potentially could suffer from this
mishandling of NULL, including the reported USDT-related one.

There are many other places where realloc()/reallocarray() is used and
NULL is always treated as an error value, but all those have guarantees
that their size is always non-zero, so those spot don't need any extra
handling.

  [0] d08ab82f59 ("libbpf: Fix double-free when linker processes empty sections")

Fixes: 999783c8bb ("libbpf: Wire up spec management and other arch-independent USDT logic")
Fixes: b63b3c490e ("libbpf: Add bpf_program__set_insns function")
Fixes: 697f104db8 ("libbpf: Support custom SEC() handlers")
Fixes: b126882672 ("libbpf: Change the order of data and text relocations.")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230711024150.1566433-1-andrii@kernel.org
This commit is contained in:
Andrii Nakryiko 2023-07-10 19:41:50 -07:00 committed by Daniel Borkmann
parent 4d496be9ca
commit 8a0260dbf6
2 changed files with 16 additions and 4 deletions
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15
tools/lib/bpf/libbpf.c
View File

@ -6161,7 +6161,11 @@ static int append_subprog_relos(struct bpf_program *main_prog, struct bpf_progra
if (main_prog == subprog)
return 0;
relos = libbpf_reallocarray(main_prog->reloc_desc, new_cnt, sizeof(*relos));
if (!relos)
/* if new count is zero, reallocarray can return a valid NULL result;
* in this case the previous pointer will be freed, so we *have to*
* reassign old pointer to the new value (even if it's NULL)
*/
if (!relos && new_cnt)
return -ENOMEM;
if (subprog->nr_reloc)
memcpy(relos + main_prog->nr_reloc, subprog->reloc_desc,
@ -8532,7 +8536,8 @@ int bpf_program__set_insns(struct bpf_program *prog,
return -EBUSY;
insns = libbpf_reallocarray(prog->insns, new_insn_cnt, sizeof(*insns));
if (!insns) {
/* NULL is a valid return from reallocarray if the new count is zero */
if (!insns && new_insn_cnt) {
pr_warn("prog '%s': failed to realloc prog code\n", prog->name);
return -ENOMEM;
}
@ -8841,7 +8846,11 @@ int libbpf_unregister_prog_handler(int handler_id)
/* try to shrink the array, but it's ok if we couldn't */
sec_defs = libbpf_reallocarray(custom_sec_defs, custom_sec_def_cnt, sizeof(*sec_defs));
if (sec_defs)
/* if new count is zero, reallocarray can return a valid NULL result;
* in this case the previous pointer will be freed, so we *have to*
* reassign old pointer to the new value (even if it's NULL)
*/
if (sec_defs || custom_sec_def_cnt == 0)
custom_sec_defs = sec_defs;
return 0;

5
tools/lib/bpf/usdt.c
View File

@ -852,8 +852,11 @@ static int bpf_link_usdt_detach(struct bpf_link *link)
* system is so exhausted on memory, it's the least of user's
* concerns, probably.
* So just do our best here to return those IDs to usdt_manager.
* Another edge case when we can legitimately get NULL is when
* new_cnt is zero, which can happen in some edge cases, so we
* need to be careful about that.
*/
if (new_free_ids) {
if (new_free_ids || new_cnt == 0) {
memcpy(new_free_ids + man->free_spec_cnt, usdt_link->spec_ids,
usdt_link->spec_cnt * sizeof(*usdt_link->spec_ids));
man->free_spec_ids = new_free_ids;