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libbpf: Implement enum value-based CO-RE relocations

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Implement two relocations of a new enumerator value-based CO-RE relocation
kind: ENUMVAL_EXISTS and ENUMVAL_VALUE.

First, ENUMVAL_EXISTS, allows to detect the presence of a named enumerator
value in the target (kernel) BTF. This is useful to do BPF helper/map/program
type support detection from BPF program side. bpf_core_enum_value_exists()
macro helper is provided to simplify built-in usage.

Second, ENUMVAL_VALUE, allows to capture enumerator integer value and relocate
it according to the target BTF, if it changes. This is useful to have
a guarantee against intentional or accidental re-ordering/re-numbering of some
of the internal (non-UAPI) enumerations, where kernel developers don't care
about UAPI backwards compatiblity concerns. bpf_core_enum_value() allows to
capture this succinctly and use correct enum values in code.

LLVM uses ldimm64 instruction to capture enumerator value-based relocations,
so add support for ldimm64 instruction patching as well.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200819194519.3375898-5-andriin@fb.com
This commit is contained in:
Andrii Nakryiko
2020-08-19 12:45:18 -07:00
committed by Andrii Nakryiko
parent cdb21b05e5
commit f89dab0903
3 changed files with 170 additions and 5 deletions
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28
src/bpf_core_read.h
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@@ -31,6 +31,12 @@ enum bpf_type_info_kind {
BPF_TYPE_SIZE = 1, /* type size in target kernel */
};
/* second argument to __builtin_preserve_enum_value() built-in */
enum bpf_enum_value_kind {
BPF_ENUMVAL_EXISTS = 0, /* enum value existence in kernel */
BPF_ENUMVAL_VALUE = 1, /* enum value value relocation */
};
#define __CORE_RELO(src, field, info) \
__builtin_preserve_field_info((src)->field, BPF_FIELD_##info)
@@ -150,6 +156,28 @@ enum bpf_type_info_kind {
#define bpf_core_type_size(type) \
__builtin_preserve_type_info(*(typeof(type) *)0, BPF_TYPE_SIZE)
/*
* Convenience macro to check that provided enumerator value is defined in
* a target kernel.
* Returns:
* 1, if specified enum type and its enumerator value are present in target
* kernel's BTF;
* 0, if no matching enum and/or enum value within that enum is found.
*/
#define bpf_core_enum_value_exists(enum_type, enum_value) \
__builtin_preserve_enum_value(*(typeof(enum_type) *)enum_value, BPF_ENUMVAL_EXISTS)
/*
* Convenience macro to get the integer value of an enumerator value in
* a target kernel.
* Returns:
* 64-bit value, if specified enum type and its enumerator value are
* present in target kernel's BTF;
* 0, if no matching enum and/or enum value within that enum is found.
*/
#define bpf_core_enum_value(enum_type, enum_value) \
__builtin_preserve_enum_value(*(typeof(enum_type) *)enum_value, BPF_ENUMVAL_VALUE)
/*
* bpf_core_read() abstracts away bpf_probe_read_kernel() call and captures
* offset relocation for source address using __builtin_preserve_access_index()