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libbpf: generalize virtual __kconfig externs and use it for USDT

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Libbpf supports single virtual __kconfig extern currently: LINUX_KERNEL_VERSION.
LINUX_KERNEL_VERSION isn't coming from /proc/kconfig.gz and is intead
customly filled out by libbpf.

This patch generalizes this approach to support more such virtual
__kconfig externs. One such extern added in this patch is
LINUX_HAS_BPF_COOKIE which is used for BPF-side USDT supporting code in
usdt.bpf.h instead of using CO-RE-based enum detection approach for
detecting bpf_get_attach_cookie() BPF helper. This allows to remove
otherwise not needed CO-RE dependency and keeps user-space and BPF-side
parts of libbpf's USDT support strictly in sync in terms of their
feature detection.

We'll use similar approach for syscall wrapper detection for
BPF_KSYSCALL() BPF-side macro in follow up patch.

Generally, currently libbpf reserves CONFIG_ prefix for Kconfig values
and LINUX_ for virtual libbpf-backed externs. In the future we might
extend the set of prefixes that are supported. This can be done without
any breaking changes, as currently any __kconfig extern with
unrecognized name is rejected.

For LINUX_xxx externs we support the normal "weak rule": if libbpf
doesn't recognize given LINUX_xxx extern but such extern is marked as
__weak, it is not rejected and defaults to zero.  This follows
CONFIG_xxx handling logic and will allow BPF applications to
opportunistically use newer libbpf virtual externs without breaking on
older libbpf versions unnecessarily.

Tested-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220714070755.3235561-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Andrii Nakryiko
2022-07-14 00:07:51 -07:00
committed by Andrii Nakryiko
parent 1648fa16b5
commit d56d93baff
2 changed files with 67 additions and 46 deletions
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16
src/usdt.bpf.h
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@@ -6,7 +6,6 @@
#include <linux/errno.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>
#include <bpf/bpf_core_read.h>
/* Below types and maps are internal implementation details of libbpf's USDT
* support and are subjects to change. Also, bpf_usdt_xxx() API helpers should
@@ -30,14 +29,6 @@
#ifndef BPF_USDT_MAX_IP_CNT
#define BPF_USDT_MAX_IP_CNT (4 * BPF_USDT_MAX_SPEC_CNT)
#endif
/* We use BPF CO-RE to detect support for BPF cookie from BPF side. This is
* the only dependency on CO-RE, so if it's undesirable, user can override
* BPF_USDT_HAS_BPF_COOKIE to specify whether to BPF cookie is supported or not.
*/
#ifndef BPF_USDT_HAS_BPF_COOKIE
#define BPF_USDT_HAS_BPF_COOKIE \
bpf_core_enum_value_exists(enum bpf_func_id___usdt, BPF_FUNC_get_attach_cookie___usdt)
#endif
enum __bpf_usdt_arg_type {
BPF_USDT_ARG_CONST,
@@ -83,15 +74,12 @@ struct {
__type(value, __u32);
} __bpf_usdt_ip_to_spec_id SEC(".maps") __weak;
/* don't rely on user's BPF code to have latest definition of bpf_func_id */
enum bpf_func_id___usdt {
BPF_FUNC_get_attach_cookie___usdt = 0xBAD, /* value doesn't matter */
};
extern const _Bool LINUX_HAS_BPF_COOKIE __kconfig;
static __always_inline
int __bpf_usdt_spec_id(struct pt_regs *ctx)
{
if (!BPF_USDT_HAS_BPF_COOKIE) {
if (!LINUX_HAS_BPF_COOKIE) {
long ip = PT_REGS_IP(ctx);
int *spec_id_ptr;