Re: [PATCH] powerpc/kprobes: Pass ppc_inst as a pointer to emulate_step() on ppc32

Le 20/05/2021 à 12:54, Naveen N. Rao a écrit :

Christophe Leroy wrote:

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Le 20/05/2021 à 09:29, Naveen N. Rao a écrit :

quoted

Trying to use a kprobe on ppc32 results in the below splat:
     BUG: Unable to handle kernel data access on read at 0x7c0802a6
     Faulting instruction address: 0xc002e9f0
     Oops: Kernel access of bad area, sig: 11 [#1]
     BE PAGE_SIZE=4K PowerPC 44x Platform
     Modules linked in:
     CPU: 0 PID: 89 Comm: sh Not tainted 5.13.0-rc1-01824-g3a81c0495fdb #7
     NIP:  c002e9f0 LR: c0011858 CTR: 00008a47
     REGS: c292fd50 TRAP: 0300   Not tainted  (5.13.0-rc1-01824-g3a81c0495fdb)
     MSR:  00009000 <EE,ME>  CR: 24002002  XER: 20000000
     DEAR: 7c0802a6 ESR: 00000000
     <snip>
     NIP [c002e9f0] emulate_step+0x28/0x324
     LR [c0011858] optinsn_slot+0x128/0x10000
     Call Trace:
      opt_pre_handler+0x7c/0xb4 (unreliable)
      optinsn_slot+0x128/0x10000
      ret_from_syscall+0x0/0x28

I remember running some kprobe tests before submitting the patch, how did I miss that ?
Is there anything special to do to activate the use of optprobes and/or to hit this bug ?

Yeah, I was surprised when I hit this. One of the requirements we have for optprobes on powerpc is that the instruction should be a compute instruction (no load/store -- emulate_update_regs() should be enough) with the exception of conditional branches. It's possible that you ended up probing an instruction that couldn't be optimized.

An easy way to confirm if a probe has been optimized is to look at kprobes/list in debugfs, and to watch out for [OPTIMIZED] flag there.

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diff --git a/arch/powerpc/kernel/optprobes.c b/arch/powerpc/kernel/optprobes.c
index cdf87086fa33a0..2bc53fa48a1b33 100644
--- a/arch/powerpc/kernel/optprobes.c
+++ b/arch/powerpc/kernel/optprobes.c

@@ -281,8 +281,12 @@ int arch_prepare_optimized_kprobe(struct optimized_kprobe *op, struct kprobe *p)

      /*
       * 3. load instruction to be emulated into relevant register, and
       */
-    temp = ppc_inst_read((struct ppc_inst *)p->ainsn.insn);
-    patch_imm_load_insns(ppc_inst_as_ulong(temp), 4, buff + TMPL_INSN_IDX);
+    if (IS_ENABLED(CONFIG_PPC64)) {
+        temp = ppc_inst_read((struct ppc_inst *)p->ainsn.insn);
+        patch_imm_load_insns(ppc_inst_as_ulong(temp), 4, buff + TMPL_INSN_IDX);
+    } else {
+        patch_imm_load_insns((unsigned long)p->ainsn.insn, 4, buff + TMPL_INSN_IDX);
+    }

It means commit https://github.com/linuxppc/linux/commit/693557ebf407a85ea400a0b501bb97687d8f4856 was not necessary and may be reverted.

Indeed, I will send a revert for it.

I'm not completely sure it is worth reverting, on an other hand it is pointless anyway to have something to convert to a u64 something that cannot be more than 32 bits on a PPC32, so now that we have ppc_inst_as_ulong() it is as good I think.

Christophe