From 222edc843c158666d2d71793b4b7d0bbb9801883 Mon Sep 17 00:00:00 2001 From: Alan Maguire Date: Thu, 26 Mar 2026 14:54:36 +0000 Subject: btf: Add BTF kind layout encoding to UAPI BTF kind layouts provide information to parse BTF kinds. By separating parsing BTF from using all the information it provides, we allow BTF to encode new features even if they cannot be used by readers. This will be helpful in particular for cases where older tools are used to parse newer BTF with kinds the older tools do not recognize; the BTF can still be parsed in such cases using kind layout. The intent is to support encoding of kind layouts optionally so that tools like pahole can add this information. For each kind, we record - length of singular element following struct btf_type - length of each of the btf_vlen() elements following - a (currently unused) flags field The ideas here were discussed at [1], [2]; hence Suggested-by: Andrii Nakryiko Signed-off-by: Alan Maguire Signed-off-by: Andrii Nakryiko Link: https://lore.kernel.org/bpf/20260326145444.2076244-2-alan.maguire@oracle.com [1] https://lore.kernel.org/bpf/CAEf4BzYjWHRdNNw4B=eOXOs_ONrDwrgX4bn=Nuc1g8JPFC34MA@mail.gmail.com/ [2] https://lore.kernel.org/bpf/20230531201936.1992188-1-alan.maguire@oracle.com/ --- include/uapi/linux/btf.h | 12 ++++++++++++ 1 file changed, 12 insertions(+) (limited to 'include/uapi/linux') diff --git a/include/uapi/linux/btf.h b/include/uapi/linux/btf.h index 266d4ffa6c07..638615ebddc2 100644 --- a/include/uapi/linux/btf.h +++ b/include/uapi/linux/btf.h @@ -8,6 +8,16 @@ #define BTF_MAGIC 0xeB9F #define BTF_VERSION 1 +/* + * BTF layout section consists of a struct btf_layout for each known + * kind at BTF encoding time. + */ +struct btf_layout { + __u8 info_sz; /* size of singular element after btf_type */ + __u8 elem_sz; /* size of each of btf_vlen(t) elements */ + __u16 flags; /* currently unused */ +}; + struct btf_header { __u16 magic; __u8 version; @@ -19,6 +29,8 @@ struct btf_header { __u32 type_len; /* length of type section */ __u32 str_off; /* offset of string section */ __u32 str_len; /* length of string section */ + __u32 layout_off; /* offset of layout section */ + __u32 layout_len; /* length of layout section */ }; /* Max # of type identifier */ -- cgit v1.2.3 From f9a80c7ce49e2a77b769264712fe2f59479b5f5a Mon Sep 17 00:00:00 2001 From: Eyal Birger Date: Tue, 31 Mar 2026 06:06:12 -0700 Subject: bpf: Clarify BPF_RB_NO_WAKEUP behavior for bpf_ringbuf_discard() Clarify bpf_ringbuf_discard() documentation for BPF_RB_NO_WAKEUP. Discarded ring buffer records are still left in the ring buffer and are only skipped when user space consumes them. This can matter when BPF_RB_NO_WAKEUP is used: a later submit relying on adaptive wakeup might not wake the consumer, because the discarded record still needs to be consumed first. Scenario: epoll_wait(rb_fd); // blocks rec = bpf_ringbuf_reserve(&rb, ...); bpf_ringbuf_discard(rec, BPF_RB_NO_WAKEUP); rec = bpf_ringbuf_reserve(&rb, ...); bpf_ringbuf_submit(rec, 0); // valid record, but no wakeup Document this in bpf_ringbuf_discard() to make the interaction between discarded records, user-space consumption, and adaptive wakeups explicit. Reported-by: Shmulik Ladkani Signed-off-by: Eyal Birger Signed-off-by: Andrii Nakryiko Link: https://lore.kernel.org/bpf/20260331130612.3762433-1-eyal.birger@gmail.com ---- v2: adapt wording per feedback from Andrii. --- include/uapi/linux/bpf.h | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'include/uapi/linux') diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h index c8d400b7680a..552bc5d9afbd 100644 --- a/include/uapi/linux/bpf.h +++ b/include/uapi/linux/bpf.h @@ -4645,7 +4645,9 @@ union bpf_attr { * Description * Discard reserved ring buffer sample, pointed to by *data*. * If **BPF_RB_NO_WAKEUP** is specified in *flags*, no notification - * of new data availability is sent. + * of new data availability is sent. Discarded records remain in + * the ring buffer until consumed by user space, so a later submit + * using adaptive wakeup might not wake up the consumer. * If **BPF_RB_FORCE_WAKEUP** is specified in *flags*, notification * of new data availability is sent unconditionally. * If **0** is specified in *flags*, an adaptive notification -- cgit v1.2.3