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libbpf: Implement type-based CO-RE relocations support

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Implement support for TYPE_EXISTS/TYPE_SIZE/TYPE_ID_LOCAL/TYPE_ID_REMOTE
relocations. These are examples of type-based relocations, as opposed to
field-based relocations supported already. The difference is that they are
calculating relocation values based on the type itself, not a field within
a struct/union.

Type-based relos have slightly different semantics when matching local types
to kernel target types, see comments in bpf_core_types_are_compat() for
details. Their behavior on failure to find target type in kernel BTF also
differs. Instead of "poisoning" relocatable instruction and failing load
subsequently in kernel, they return 0 (which is rarely a valid return result,
so user BPF code can use that to detect success/failure of the relocation and
deal with it without extra "guarding" relocations). Also, it's always possible
to check existence of the type in target kernel with TYPE_EXISTS relocation,
similarly to a field-based FIELD_EXISTS.

TYPE_ID_LOCAL relocation is a bit special in that it always succeeds (barring
any libbpf/Clang bugs) and resolved to BTF ID using **local** BTF info of BPF
program itself. Tests in subsequent patches demonstrate the usage and
semantics of new relocations.

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/20200819194519.3375898-2-andriin@fb.com
This commit is contained in:
Andrii Nakryiko
2020-08-19 12:45:15 -07:00
committed by Andrii Nakryiko
parent a734ef0803
commit cdb21b05e5
3 changed files with 263 additions and 24 deletions
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52
src/bpf_core_read.h
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@@ -19,6 +19,18 @@ enum bpf_field_info_kind {
BPF_FIELD_RSHIFT_U64 = 5,
};
/* second argument to __builtin_btf_type_id() built-in */
enum bpf_type_id_kind {
BPF_TYPE_ID_LOCAL = 0, /* BTF type ID in local program */
BPF_TYPE_ID_TARGET = 1, /* BTF type ID in target kernel */
};
/* second argument to __builtin_preserve_type_info() built-in */
enum bpf_type_info_kind {
BPF_TYPE_EXISTS = 0, /* type existence in target kernel */
BPF_TYPE_SIZE = 1, /* type size in target kernel */
};
#define __CORE_RELO(src, field, info) \
__builtin_preserve_field_info((src)->field, BPF_FIELD_##info)
@@ -94,12 +106,50 @@ enum bpf_field_info_kind {
__builtin_preserve_field_info(field, BPF_FIELD_EXISTS)
/*
* Convenience macro to get byte size of a field. Works for integers,
* Convenience macro to get the byte size of a field. Works for integers,
* struct/unions, pointers, arrays, and enums.
*/
#define bpf_core_field_size(field) \
__builtin_preserve_field_info(field, BPF_FIELD_BYTE_SIZE)
/*
* Convenience macro to get BTF type ID of a specified type, using a local BTF
* information. Return 32-bit unsigned integer with type ID from program's own
* BTF. Always succeeds.
*/
#define bpf_core_type_id_local(type) \
__builtin_btf_type_id(*(typeof(type) *)0, BPF_TYPE_ID_LOCAL)
/*
* Convenience macro to get BTF type ID of a target kernel's type that matches
* specified local type.
* Returns:
* - valid 32-bit unsigned type ID in kernel BTF;
* - 0, if no matching type was found in a target kernel BTF.
*/
#define bpf_core_type_id_kernel(type) \
__builtin_btf_type_id(*(typeof(type) *)0, BPF_TYPE_ID_TARGET)
/*
* Convenience macro to check that provided named type
* (struct/union/enum/typedef) exists in a target kernel.
* Returns:
* 1, if such type is present in target kernel's BTF;
* 0, if no matching type is found.
*/
#define bpf_core_type_exists(type) \
__builtin_preserve_type_info(*(typeof(type) *)0, BPF_TYPE_EXISTS)
/*
* Convenience macro to get the byte size of a provided named type
* (struct/union/enum/typedef) in a target kernel.
* Returns:
* >= 0 size (in bytes), if type is present in target kernel's BTF;
* 0, if no matching type is found.
*/
#define bpf_core_type_size(type) \
__builtin_preserve_type_info(*(typeof(type) *)0, BPF_TYPE_SIZE)
/*
* bpf_core_read() abstracts away bpf_probe_read_kernel() call and captures
* offset relocation for source address using __builtin_preserve_access_index()