6.6-stable review patch. If anyone has any objections, please let me know.
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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