Merge branch 'bpf: Add bpf_task_pt_regs() helper'

Daniel Xu says:

====================

The motivation behind this helper is to access userspace pt_regs in a
kprobe handler.

uprobe's ctx is the userspace pt_regs. kprobe's ctx is the kernelspace
pt_regs. bpf_task_pt_regs() allows accessing userspace pt_regs in a
kprobe handler. The final case (kernelspace pt_regs in uprobe) is
pretty rare (usermode helper) so I think that can be solved later if
necessary.

More concretely, this helper is useful in doing BPF-based DWARF stack
unwinding. Currently the kernel can only do framepointer based stack
unwinds for userspace code. This is because the DWARF state machines are
too fragile to be computed in kernelspace [0]. The idea behind
DWARF-based stack unwinds w/ BPF is to copy a chunk of the userspace
stack (while in prog context) and send it up to userspace for unwinding
(probably with libunwind) [1]. This would effectively enable profiling
applications with -fomit-frame-pointer using kprobes and uprobes.

[0]: https://lkml.org/lkml/2012/2/10/356
[1]: https://github.com/danobi/bpf-dwarf-walk

Changes from v1:
- Conwolidate BTF_ID decls for task_struct
- Enable bpf_get_current_task_btf() for all prog types
- Enable bpf_task_pt_regs() for all prog types
- Use ASSERT_* macros instead of CHECK
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 12 insertions, 0 deletions

diff --git a/include/linux/btf_ids.h b/include/linux/btf_ids.h
index bed4b9964581..93d881ab0d48 100644
--- a/include/linux/btf_ids.h
+++ b/include/linux/btf_ids.h
@@ -82,6 +82,9 @@ __BTF_ID_LIST(name, globl)
 #define BTF_ID_LIST_SINGLE(name, prefix, typename)	\
 	BTF_ID_LIST(name) \
 	BTF_ID(prefix, typename)
+#define BTF_ID_LIST_GLOBAL_SINGLE(name, prefix, typename) \
+	BTF_ID_LIST_GLOBAL(name) \
+	BTF_ID(prefix, typename)
 
 /*
  * The BTF_ID_UNUSED macro defines 4 zero bytes.
@@ -185,4 +188,6 @@ MAX_BTF_SOCK_TYPE,
 extern u32 btf_sock_ids[];
 #endif
 
+extern u32 btf_task_struct_ids[];
+
 #endif
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 191f0b286ee3..791f31dd0abe 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -4871,6 +4871,12 @@ union bpf_attr {
  * 	Return
  *		Value specified by user at BPF link creation/attachment time
  *		or 0, if it was not specified.
+ *
+ * long bpf_task_pt_regs(struct task_struct *task)
+ *	Description
+ *		Get the struct pt_regs associated with **task**.
+ *	Return
+ *		A pointer to struct pt_regs.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5048,6 +5054,7 @@ union bpf_attr {
 	FN(timer_cancel),		\
 	FN(get_func_ip),		\
 	FN(get_attach_cookie),		\
+	FN(task_pt_regs),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper