Re: [PATCH][v2] uprobes/x86: emulate push insns for uprobe on x86

[Yonghong Song]

On 11/13/17 4:59 AM, Oleg Nesterov wrote:
The patch looks good to me, but I have a question because I know nothing
about insn encoding,

On 11/10, Yonghong Song wrote:

+static int push_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
+{
+       u8 opc1 = OPCODE1(insn), reg_offset = 0;
+
+       if (opc1 < 0x50 || opc1 > 0x57)
+               return -ENOSYS;
+
+       if (insn->length > 2)
+               return -ENOSYS;
+       if (insn->length == 2) {
+               /* only support rex_prefix 0x41 (x64 only) */
+#ifdef CONFIG_X86_64
+               if (insn->rex_prefix.nbytes != 1 ||
+                   insn->rex_prefix.bytes[0] != 0x41)
+                       return -ENOSYS;
+
+               auprobe->push.ilen = 2;

and the "else" branch does

                auprobe->push.ilen = 1;

you could add
                auprobe->push.ilen = insn->length;

at the end of push_setup_xol_ops() instead, but this is minor/cosmetic,

Will make this change in the next revision.



+               switch (opc1) {
+               case 0x50:
+                       reg_offset = offsetof(struct pt_regs, r8);
+                       break;
+               case 0x51:
+                       reg_offset = offsetof(struct pt_regs, r9);
+                       break;
+               case 0x52:
+                       reg_offset = offsetof(struct pt_regs, r10);
+                       break;
+               case 0x53:
+                       reg_offset = offsetof(struct pt_regs, r11);
+                       break;
+               case 0x54:
+                       reg_offset = offsetof(struct pt_regs, r12);
+                       break;
+               case 0x55:
+                       reg_offset = offsetof(struct pt_regs, r13);
+                       break;
+               case 0x56:
+                       reg_offset = offsetof(struct pt_regs, r14);
+                       break;
+               case 0x57:
+                       reg_offset = offsetof(struct pt_regs, r15);
+                       break;
+               }
+#else
+               return -ENOSYS;
+#endif

OK, but shouldn't we also return ENOSYS if CONFIG_X86_64=y but the probed task 
is 32bit?

Just tested with a 32bit app on x86 box and segfaults. Yes, we would 
need to return ENOSYS if the app is 32bit on 64bit system. I may not
be worthwhile to emulate this uncommon case.

I will use mmap_is_ia32 or a variant to test whether the app is
32bit or not. Please let me know whether this is correct approach or not.


Or in this case uprobe_init_insn(x86_64 => false) should fail and 
push_setup_xol_ops()
won't be called?

Oleg.