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bpf: Add bpf_task_pt_regs() helper

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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

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/e2718ced2d51ef4268590ab8562962438ab82815.1629772842.git.dxu@dxuuu.xyz
This commit is contained in:
Daniel Xu 2021-08-23 19:43:49 -07:00 • committed by Alexei Starovoitov
parent a396eda551
commit dd6e10fbd9
4 changed files with 36 additions and 0 deletions
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7
tools/include/uapi/linux/bpf.h
View file

@ -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