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authorKumar Kartikeya Dwivedi <memxor@gmail.com>2022-04-25 00:48:51 +0300
committerAlexei Starovoitov <ast@kernel.org>2022-04-26 06:26:05 +0300
commitc0a5a21c25f37c9fd7b36072f9968cdff1e4aa13 (patch)
treed96d885e62415accde125773c587fedfc1f759d8 /tools/include
parent8f14852e89113d738c99c375b4c8b8b7e1073df1 (diff)
downloadlinux-c0a5a21c25f37c9fd7b36072f9968cdff1e4aa13.tar.xz
bpf: Allow storing referenced kptr in map
Extending the code in previous commits, introduce referenced kptr support, which needs to be tagged using 'kptr_ref' tag instead. Unlike unreferenced kptr, referenced kptr have a lot more restrictions. In addition to the type matching, only a newly introduced bpf_kptr_xchg helper is allowed to modify the map value at that offset. This transfers the referenced pointer being stored into the map, releasing the references state for the program, and returning the old value and creating new reference state for the returned pointer. Similar to unreferenced pointer case, return value for this case will also be PTR_TO_BTF_ID_OR_NULL. The reference for the returned pointer must either be eventually released by calling the corresponding release function, otherwise it must be transferred into another map. It is also allowed to call bpf_kptr_xchg with a NULL pointer, to clear the value, and obtain the old value if any. BPF_LDX, BPF_STX, and BPF_ST cannot access referenced kptr. A future commit will permit using BPF_LDX for such pointers, but attempt at making it safe, since the lifetime of object won't be guaranteed. There are valid reasons to enforce the restriction of permitting only bpf_kptr_xchg to operate on referenced kptr. The pointer value must be consistent in face of concurrent modification, and any prior values contained in the map must also be released before a new one is moved into the map. To ensure proper transfer of this ownership, bpf_kptr_xchg returns the old value, which the verifier would require the user to either free or move into another map, and releases the reference held for the pointer being moved in. In the future, direct BPF_XCHG instruction may also be permitted to work like bpf_kptr_xchg helper. Note that process_kptr_func doesn't have to call check_helper_mem_access, since we already disallow rdonly/wronly flags for map, which is what check_map_access_type checks, and we already ensure the PTR_TO_MAP_VALUE refers to kptr by obtaining its off_desc, so check_map_access is also not required. Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/20220424214901.2743946-4-memxor@gmail.com
Diffstat (limited to 'tools/include')
-rw-r--r--tools/include/uapi/linux/bpf.h12
1 files changed, 12 insertions, 0 deletions
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index d14b10b85e51..444fe6f1cf35 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -5143,6 +5143,17 @@ union bpf_attr {
* The **hash_algo** is returned on success,
* **-EOPNOTSUP** if the hash calculation failed or **-EINVAL** if
* invalid arguments are passed.
+ *
+ * void *bpf_kptr_xchg(void *map_value, void *ptr)
+ * Description
+ * Exchange kptr at pointer *map_value* with *ptr*, and return the
+ * old value. *ptr* can be NULL, otherwise it must be a referenced
+ * pointer which will be released when this helper is called.
+ * Return
+ * The old value of kptr (which can be NULL). The returned pointer
+ * if not NULL, is a reference which must be released using its
+ * corresponding release function, or moved into a BPF map before
+ * program exit.
*/
#define __BPF_FUNC_MAPPER(FN) \
FN(unspec), \
@@ -5339,6 +5350,7 @@ union bpf_attr {
FN(copy_from_user_task), \
FN(skb_set_tstamp), \
FN(ima_file_hash), \
+ FN(kptr_xchg), \
/* */
/* integer value in 'imm' field of BPF_CALL instruction selects which helper