[PATCH 6.1 124/197] powerpc/64: Set task pt_regs->link to the LR value on scv entry

Greg Kroah-Hartman
2024-02-20T21:04:26Z

From: Naveen N Rao <naveen@kernel.org>

commit aad98efd0b121f63a2e1c221dcb4d4850128c697 upstream.

Nysal reported that userspace backtraces are missing in offcputime bcc
tool. As an example:
$ sudo ./bcc/tools/offcputime.py -uU
Tracing off-CPU time (us) of user threads by user stack... Hit Ctrl-C to end.

^C
write
- python (9107)
8

write
- sudo (9105)
9

mmap
- python (9107)
16

clock_nanosleep
- multipathd (697)
3001604

The offcputime bcc tool attaches a bpf program to a kprobe on
finish_task_switch(), which is usually hit on a syscall from userspace.
With the switch to system call vectored, we started setting
pt_regs->link to zero. This is because system call vectored behaves like
a function call with LR pointing to the system call return address, and
with no modification to SRR0/SRR1. The LR value does indicate our next
instruction, so it is being saved as pt_regs->nip, and pt_regs->link is
being set to zero. This is not a problem by itself, but BPF uses perf
callchain infrastructure for capturing stack traces, and that stores LR
as the second entry in the stack trace. perf has code to cope with the
second entry being zero, and skips over it. However, generic userspace
unwinders assume that a zero entry indicates end of the stack trace,
resulting in a truncated userspace stack trace.

Rather than fixing all userspace unwinders to ignore/skip past the
second entry, store the real LR value in pt_regs->link so that there
continues to be a valid, though duplicate entry in the stack trace.

With this change:
$ sudo ./bcc/tools/offcputime.py -uU
Tracing off-CPU time (us) of user threads by user stack... Hit Ctrl-C to end.

^C
write
write
[unknown]
[unknown]
[unknown]
[unknown]
[unknown]
PyObject_VectorcallMethod
[unknown]
[unknown]
PyObject_CallOneArg
PyFile_WriteObject
PyFile_WriteString
[unknown]
[unknown]
PyObject_Vectorcall
_PyEval_EvalFrameDefault
PyEval_EvalCode
[unknown]
[unknown]
[unknown]
_PyRun_SimpleFileObject
_PyRun_AnyFileObject
Py_RunMain
[unknown]
Py_BytesMain
[unknown]
__libc_start_main
- python (1293)
7

write
write
[unknown]
sudo_ev_loop_v1
sudo_ev_dispatch_v1
[unknown]
[unknown]
[unknown]
[unknown]
__libc_start_main
- sudo (1291)
7

syscall
syscall
bpf_open_perf_buffer_opts
[unknown]
[unknown]
[unknown]
[unknown]
_PyObject_MakeTpCall
PyObject_Vectorcall
_PyEval_EvalFrameDefault
PyEval_EvalCode
[unknown]
[unknown]
[unknown]
_PyRun_SimpleFileObject
_PyRun_AnyFileObject
Py_RunMain
[unknown]
Py_BytesMain
[unknown]
__libc_start_main
- python (1293)
11

clock_nanosleep
clock_nanosleep
nanosleep
sleep
[unknown]
[unknown]
__clone
- multipathd (698)
3001661

Fixes: 7fa95f9adaee ("powerpc/64s: system call support for scv/rfscv instructions")
Cc: stable@vger.kernel.org
Reported-by: "Nysal Jan K.A" <nysal@linux.ibm.com>
Signed-off-by: Naveen N Rao <naveen@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240202154316.395276-1-naveen@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/powerpc/kernel/interrupt_64.S | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/powerpc/kernel/interrupt_64.S
+++ b/arch/powerpc/kernel/interrupt_64.S
@@ -52,7 +52,8 @@ _ASM_NOKPROBE_SYMBOL(system_call_vectore
mr r10,r1
ld r1,PACAKSAVE(r13)
std r10,0(r1)
- std r11,_NIP(r1)
+ std r11,_LINK(r1)
+ std r11,_NIP(r1) /* Saved LR is also the next instruction */
std r12,_MSR(r1)
std r0,GPR0(r1)
std r10,GPR1(r1)
@@ -70,7 +71,6 @@ _ASM_NOKPROBE_SYMBOL(system_call_vectore
std r9,GPR13(r1)
SAVE_NVGPRS(r1)
std r11,_XER(r1)
- std r11,_LINK(r1)
std r11,_CTR(r1)

li r11,\trapnr