From a5f7a5bb3b7f28ba7e4fa246775b29a0e5537255 Mon Sep 17 00:00:00 2001 From: Jinke Han Date: Tue, 8 Sep 2026 15:37:42 +0800 Subject: x86/kprobes: Fix crash when probing CS CALL instructions When using eBPF to probe CS CALL instructions within a function, a crash can be triggered. The eBPF tool probes offset 257 of the __hrtimer_run_queues() function: <__hrtimer_run_queues+249>: nopl 0x0(%rax,%rax,1) <__hrtimer_run_queues+254>: mov %r14,%rdi <__hrtimer_run_queues+257>: cs call <__x86_indirect_thunk_r12> <__hrtimer_run_queues+263>: mov %eax,%r12d <__hrtimer_run_queues+266>: xchg %ax,%ax <__hrtimer_run_queues+268>: mov %r13,%rdi Which triggers this crash: BUG: unable to handle page fault for address: 00000000000f41c9 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page PGD 0 P4D 0 Oops: 0002 [#1] SMP NOPTI CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P RIP: 0010:__hrtimer_run_queues+0x106/0x230 Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is at the 6th byte of the above CS CALL instruction. Since the CS CALL instruction occupies 6 bytes, the exception occurred in the middle of that call instruction. The root cause is that when using eBPF tools to probe in the middle of a function, a kprobe with INT3 is used as the underlying implementation. During single-step emulation of the original CALL instruction, int3_emulate_call() assumes that the probed CALL instruction is 5 bytes long. However, the actual CS-prefixed CALL instruction occupies 6 bytes, so it constructs an incorrect exception return address. When the CPU returns from the kprobe handler, the next instruction to be executed is at the address of the last byte of that CS CALL instruction. Coincidentally, starting from that address, the CPU fetches and decodes a completely different instruction, which ultimately triggers a kernel crash. Fix the issue by using the actual instruction length obtained from the instruction decoder when constructing the exception return address, rather than relying on the hardcoded CALL_INSN_SIZE macro. [ mingo: Refined the changelog ] Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step") Suggested-by: Masami Hiramatsu (Google) Signed-off-by: Jinke Han Signed-off-by: Ingo Molnar Reviewed-by: Masami Hiramatsu (Google) Acked-by: Yafang Shao Acked-by: Borislav Petkov Cc: Peter Zijlstra Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000 --- Documentation/bpf/map_array.rst | 263 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 263 insertions(+) create mode 100644 Documentation/bpf/map_array.rst (limited to 'Documentation/bpf/map_array.rst') diff --git a/Documentation/bpf/map_array.rst b/Documentation/bpf/map_array.rst new file mode 100644 index 000000000..fa56ff751 --- /dev/null +++ b/Documentation/bpf/map_array.rst @@ -0,0 +1,263 @@ +.. SPDX-License-Identifier: GPL-2.0-only +.. Copyright (C) 2022 Red Hat, Inc. + +================================================ +BPF_MAP_TYPE_ARRAY and BPF_MAP_TYPE_PERCPU_ARRAY +================================================ + +.. note:: + - ``BPF_MAP_TYPE_ARRAY`` was introduced in kernel version 3.19 + - ``BPF_MAP_TYPE_PERCPU_ARRAY`` was introduced in version 4.6 + +``BPF_MAP_TYPE_ARRAY`` and ``BPF_MAP_TYPE_PERCPU_ARRAY`` provide generic array +storage. The key type is an unsigned 32-bit integer (4 bytes) and the map is +of constant size. The size of the array is defined in ``max_entries`` at +creation time. All array elements are pre-allocated and zero initialized when +created. ``BPF_MAP_TYPE_PERCPU_ARRAY`` uses a different memory region for each +CPU whereas ``BPF_MAP_TYPE_ARRAY`` uses the same memory region. The value +stored can be of any size for ``BPF_MAP_TYPE_ARRAY`` and not more than +``PCPU_MIN_UNIT_SIZE`` (32 kB) for ``BPF_MAP_TYPE_PERCPU_ARRAY``. All +array elements are aligned to 8 bytes. + +Since kernel 5.5, memory mapping may be enabled for ``BPF_MAP_TYPE_ARRAY`` by +setting the flag ``BPF_F_MMAPABLE``. The map definition is page-aligned and +starts on the first page. Sufficient page-sized and page-aligned blocks of +memory are allocated to store all array values, starting on the second page, +which in some cases will result in over-allocation of memory. The benefit of +using this is increased performance and ease of use since userspace programs +would not be required to use helper functions to access and mutate data. + +Usage +===== + +Kernel BPF +---------- + +bpf_map_lookup_elem() +~~~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + void *bpf_map_lookup_elem(struct bpf_map *map, const void *key) + +Array elements can be retrieved using the ``bpf_map_lookup_elem()`` helper. +This helper returns a pointer into the array element, so to avoid data races +with userspace reading the value, the user must use primitives like +``__sync_fetch_and_add()`` when updating the value in-place. + +bpf_map_update_elem() +~~~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags) + +Array elements can be updated using the ``bpf_map_update_elem()`` helper. + +``bpf_map_update_elem()`` returns 0 on success, or negative error in case of +failure. + +Since the array is of constant size, ``bpf_map_delete_elem()`` is not supported. +To clear an array element, you may use ``bpf_map_update_elem()`` to insert a +zero value to that index. + +Per CPU Array +------------- + +Values stored in ``BPF_MAP_TYPE_ARRAY`` can be accessed by multiple programs +across different CPUs. To restrict storage to a single CPU, you may use a +``BPF_MAP_TYPE_PERCPU_ARRAY``. + +When using a ``BPF_MAP_TYPE_PERCPU_ARRAY`` the ``bpf_map_update_elem()`` and +``bpf_map_lookup_elem()`` helpers automatically access the slot for the current +CPU. + +bpf_map_lookup_percpu_elem() +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + void *bpf_map_lookup_percpu_elem(struct bpf_map *map, const void *key, u32 cpu) + +The ``bpf_map_lookup_percpu_elem()`` helper can be used to lookup the array +value for a specific CPU. Returns value on success , or ``NULL`` if no entry was +found or ``cpu`` is invalid. + +Concurrency +----------- + +Since kernel version 5.1, the BPF infrastructure provides ``struct bpf_spin_lock`` +to synchronize access. + +Userspace +--------- + +Access from userspace uses libbpf APIs with the same names as above, with +the map identified by its ``fd``. + +Examples +======== + +Please see the ``tools/testing/selftests/bpf`` directory for functional +examples. The code samples below demonstrate API usage. + +Kernel BPF +---------- + +This snippet shows how to declare an array in a BPF program. + +.. code-block:: c + + struct { + __uint(type, BPF_MAP_TYPE_ARRAY); + __type(key, u32); + __type(value, long); + __uint(max_entries, 256); + } my_map SEC(".maps"); + + +This example BPF program shows how to access an array element. + +.. code-block:: c + + int bpf_prog(struct __sk_buff *skb) + { + struct iphdr ip; + int index; + long *value; + + if (bpf_skb_load_bytes(skb, ETH_HLEN, &ip, sizeof(ip)) < 0) + return 0; + + index = ip.protocol; + value = bpf_map_lookup_elem(&my_map, &index); + if (value) + __sync_fetch_and_add(value, skb->len); + + return 0; + } + +Userspace +--------- + +BPF_MAP_TYPE_ARRAY +~~~~~~~~~~~~~~~~~~ + +This snippet shows how to create an array, using ``bpf_map_create_opts`` to +set flags. + +.. code-block:: c + + #include + #include + + int create_array() + { + int fd; + LIBBPF_OPTS(bpf_map_create_opts, opts, .map_flags = BPF_F_MMAPABLE); + + fd = bpf_map_create(BPF_MAP_TYPE_ARRAY, + "example_array", /* name */ + sizeof(__u32), /* key size */ + sizeof(long), /* value size */ + 256, /* max entries */ + &opts); /* create opts */ + return fd; + } + +This snippet shows how to initialize the elements of an array. + +.. code-block:: c + + int initialize_array(int fd) + { + __u32 i; + long value; + int ret; + + for (i = 0; i < 256; i++) { + value = i; + ret = bpf_map_update_elem(fd, &i, &value, BPF_ANY); + if (ret < 0) + return ret; + } + + return ret; + } + +This snippet shows how to retrieve an element value from an array. + +.. code-block:: c + + int lookup(int fd) + { + __u32 index = 42; + long value; + int ret; + + ret = bpf_map_lookup_elem(fd, &index, &value); + if (ret < 0) + return ret; + + /* use value here */ + assert(value == 42); + + return ret; + } + +BPF_MAP_TYPE_PERCPU_ARRAY +~~~~~~~~~~~~~~~~~~~~~~~~~ + +This snippet shows how to initialize the elements of a per CPU array. + +.. code-block:: c + + int initialize_array(int fd) + { + int ncpus = libbpf_num_possible_cpus(); + long values[ncpus]; + __u32 i, j; + int ret; + + for (i = 0; i < 256 ; i++) { + for (j = 0; j < ncpus; j++) + values[j] = i; + ret = bpf_map_update_elem(fd, &i, &values, BPF_ANY); + if (ret < 0) + return ret; + } + + return ret; + } + +This snippet shows how to access the per CPU elements of an array value. + +.. code-block:: c + + int lookup(int fd) + { + int ncpus = libbpf_num_possible_cpus(); + __u32 index = 42, j; + long values[ncpus]; + int ret; + + ret = bpf_map_lookup_elem(fd, &index, &values); + if (ret < 0) + return ret; + + for (j = 0; j < ncpus; j++) { + /* Use per CPU value here */ + assert(values[j] == 42); + } + + return ret; + } + +Semantics +========= + +As shown in the example above, when accessing a ``BPF_MAP_TYPE_PERCPU_ARRAY`` +in userspace, each value is an array with ``ncpus`` elements. + +When calling ``bpf_map_update_elem()`` the flag ``BPF_NOEXIST`` can not be used +for these maps. -- cgit v1.3.1