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bpf: BTF support for ksyms #1
Commits on Sep 4, 2020
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Pseudo_btf_id is a type of ld_imm insn that associates a btf_id to a
ksym so that further dereferences on the ksym can use the BTF info to validate accesses. Internally, when seeing a pseudo_btf_id ld insn, the verifier reads the btf_id stored in the insn[0]'s imm field and marks the dst_reg as PTR_TO_BTF_ID. The btf_id points to a VAR_KIND, which is encoded in btf_vminux by pahole. If the VAR is not of a struct type, the dst reg will be marked as PTR_TO_MEM instead of PTR_TO_BTF_ID and the mem_size is resolved to the size of the VAR's type. From the VAR btf_id, the verifier can also read the address of the ksym's corresponding kernel var from kallsyms and use that to fill dst_reg. Therefore, the proper functionality of pseudo_btf_id depends on (1) kallsyms and (2) the encoding of kernel global VARs in pahole, which should be available since pahole v1.18. Signed-off-by: Hao Luo <haoluo@google.com> --- include/linux/bpf_verifier.h | 4 ++ include/linux/btf.h | 15 +++++ include/uapi/linux/bpf.h | 38 ++++++++--- kernel/bpf/btf.c | 15 ----- kernel/bpf/verifier.c | 112 ++++++++++++++++++++++++++++++--- tools/include/uapi/linux/bpf.h | 38 ++++++++--- 6 files changed, 182 insertions(+), 40 deletions(-)
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If a ksym is defined with a type, libbpf will try to find the ksym's btf
information from kernel btf. If a valid btf entry for the ksym is found, libbpf can pass in the found btf id to the verifier, which validates the ksym's type and value. Typeless ksyms (i.e. those defined as 'void') will not have such btf_id, but it has the symbol's address (read from kallsyms) and its value is treated as a raw pointer. Signed-off-by: Hao Luo <haoluo@google.com> --- tools/lib/bpf/libbpf.c | 116 ++++++++++++++++++++++++++++++++++++----- 1 file changed, 102 insertions(+), 14 deletions(-)
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Selftests for typed ksyms. Tests two types of ksyms: one is a struct,
the other is a plain int. This tests two paths in the kernel. Struct ksyms will be converted into PTR_TO_BTF_ID by the verifier while int typed ksyms will be converted into PTR_TO_MEM. Signed-off-by: Hao Luo <haoluo@google.com> --- .../testing/selftests/bpf/prog_tests/ksyms.c | 31 +++------ .../selftests/bpf/prog_tests/ksyms_btf.c | 63 +++++++++++++++++++ .../selftests/bpf/progs/test_ksyms_btf.c | 23 +++++++ tools/testing/selftests/bpf/trace_helpers.c | 26 ++++++++ tools/testing/selftests/bpf/trace_helpers.h | 4 ++ 5 files changed, 123 insertions(+), 24 deletions(-) create mode 100644 tools/testing/selftests/bpf/prog_tests/ksyms_btf.c create mode 100644 tools/testing/selftests/bpf/progs/test_ksyms_btf.c
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Add bpf_per_cpu_ptr() to help bpf programs access percpu vars.
bpf_per_cpu_ptr() has the same semantic as per_cpu_ptr() in the kernel except that it may return NULL. This happens when the cpu parameter is out of range. So the caller must check the returned value. Acked-by: Andrii Nakryiko <andriin@fb.com> Signed-off-by: Hao Luo <haoluo@google.com> --- include/linux/bpf.h | 3 ++ include/linux/btf.h | 11 ++++++ include/uapi/linux/bpf.h | 17 +++++++++ kernel/bpf/btf.c | 10 ------ kernel/bpf/verifier.c | 66 +++++++++++++++++++++++++++++++--- kernel/trace/bpf_trace.c | 18 ++++++++++ tools/include/uapi/linux/bpf.h | 17 +++++++++ 7 files changed, 128 insertions(+), 14 deletions(-)
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Add bpf_this_cpu_ptr() to help access percpu var on this cpu. This
helper always returns a valid pointer, therefore no need to check returned value for NULL. Also note that all programs run with preemption disabled, which means that the returned pointer is stable during all the execution of the program. Signed-off-by: Hao Luo <haoluo@google.com> --- include/linux/bpf.h | 1 + include/uapi/linux/bpf.h | 14 ++++++++++++++ kernel/bpf/verifier.c | 10 +++++++--- kernel/trace/bpf_trace.c | 14 ++++++++++++++ tools/include/uapi/linux/bpf.h | 14 ++++++++++++++ 5 files changed, 50 insertions(+), 3 deletions(-)
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Test bpf_per_cpu_ptr() and bpf_this_cpu_ptr(). Test two paths in the
kernel. If the base pointer points to a struct, the returned reg is of type PTR_TO_BTF_ID. Direct pointer dereference can be applied on the returned variable. If the base pointer isn't a struct, the returned reg is of type PTR_TO_MEM, which also supports direct pointer dereference. Acked-by: Andrii Nakryiko <andriin@fb.com> Signed-off-by: Hao Luo <haoluo@google.com> --- .../selftests/bpf/prog_tests/ksyms_btf.c | 10 +++++++ .../selftests/bpf/progs/test_ksyms_btf.c | 26 +++++++++++++++++++ 2 files changed, 36 insertions(+)
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