[POC 07/12] x86-64: rai: implement _rai_load
From: Rasmus Villemoes
Date: Wed Oct 17 2018 - 18:33:52 EST
This implements the simplest of the rai_* operations, loading a
value. For load of an 8-byte value, I believe we do need to keep room
for a movabs, since there's no guarantee the final value can be loaded
with as an imm32 or using a %rip-relative leaq.
It wouldn't hurt to add some sanity checking in rai_patch_one, e.g. at
least check that the immediate we are replacing is the dummy 0x12345678
we used in the .rai_templ section.
That the patching works can be seen in a quick virtme session. gdb on
vmlinux and /proc/kcore shows
(gdb) x/16i rai_proc_show
0xffffffff8108c120 <rai_proc_show>: mov $0xffffffff81fd9ad4,%rsi
0xffffffff8108c127 <rai_proc_show+7>: jmpq 0xffffffff819652e9
0xffffffff8108c12c <rai_proc_show+12>: nop
0xffffffff8108c12d <rai_proc_show+13>: nop
0xffffffff8108c12e <rai_proc_show+14>: nop
0xffffffff8108c12f <rai_proc_show+15>: nop
0xffffffff8108c130 <rai_proc_show+16>: nop
0xffffffff8108c131 <rai_proc_show+17>: jmpq 0xffffffff819652f5
0xffffffff8108c136 <rai_proc_show+22>: jmpq 0xffffffff81965300
0xffffffff8108c13b <rai_proc_show+27>: callq 0xffffffff81238bb0 <seq_printf>
0xffffffff8108c140 <rai_proc_show+32>: mov $0xffffffffffffffff,%rax
0xffffffff8108c147 <rai_proc_show+39>: mov %rax,0x17b228a(%rip) # 0xffffffff8283e3d8 <three>
0xffffffff8108c14e <rai_proc_show+46>: mov %eax,0x17b228c(%rip) # 0xffffffff8283e3e0 <two>
0xffffffff8108c154 <rai_proc_show+52>: mov %eax,0x17b228a(%rip) # 0xffffffff8283e3e4 <one>
0xffffffff8108c15a <rai_proc_show+58>: xor %eax,%eax
0xffffffff8108c15c <rai_proc_show+60>: retq
(gdb) x/16i 0xffffffff96e8c120
0xffffffff96e8c120: mov $0xffffffff97dd9ad4,%rsi
0xffffffff96e8c127: movabs $0x3,%r8
0xffffffff96e8c131: mov $0x2,%ecx
0xffffffff96e8c136: mov $0x1,%edx
0xffffffff96e8c13b: callq 0xffffffff97038bb0
0xffffffff96e8c140: mov $0xffffffffffffffff,%rax
0xffffffff96e8c147: mov %rax,0x17b228a(%rip) # 0xffffffff9863e3d8
0xffffffff96e8c14e: mov %eax,0x17b228c(%rip) # 0xffffffff9863e3e0
0xffffffff96e8c154: mov %eax,0x17b228a(%rip) # 0xffffffff9863e3e4
0xffffffff96e8c15a: xor %eax,%eax
0xffffffff96e8c15c: retq
0xffffffff96e8c15d: nopl (%rax)
0xffffffff96e8c160: push %rbx
0xffffffff96e8c161: mov $0xffffffff9804c240,%rdi
0xffffffff96e8c168: mov $0xffffffff97e9fccc,%rbx
0xffffffff96e8c16f: callq 0xffffffff9776b230
where we also see that gcc chooses the destination registers rather
intelligently. As expected, repeated "cat /proc/rai" continues to print
"one: 1, two: 2, three: 3".
Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
---
arch/x86/include/asm/rai.S | 42 +++++++++++++++++++++++++++++++++++++-
arch/x86/include/asm/rai.h | 30 ++++++++++++++++++++++++++-
arch/x86/kernel/rai.c | 18 ++++++++++++++++
3 files changed, 88 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/rai.S b/arch/x86/include/asm/rai.S
index 253d27453416..f42cdd8db876 100644
--- a/arch/x86/include/asm/rai.S
+++ b/arch/x86/include/asm/rai.S
@@ -8,11 +8,51 @@
.long \templ_end - \templ
.long \thunk - .
.endm
-
+
.macro rai_entry_pad start end
.ifgt STRUCT_RAI_ENTRY_SIZE-(\end-\start)
.skip STRUCT_RAI_ENTRY_SIZE-(\end-\start), 0x00
.endif
.endm
+.macro rai_load dst, var, type
+ .pushsection .rai_templ, "aw"
+10:
+ .ifeq \type - RAI_LOAD_8
+ movabs $0x1234567812345678, \dst
+ .else
+ mov $0x12345678, \dst
+ .endif
+11:
+ .popsection
+
+ /* Even if the mov \var, \dst is short enough to fit in the
+ * space we reserve in .text, we still need the thunk for when
+ * we do the immediate patching. */
+ .pushsection .text.rai_thunk, "ax"
+20:
+ mov \var(%rip), \dst
+ jmp 32f
+21:
+ .popsection
+
+ /* The part that goes into .text */
+30:
+ /* silence objtool by actually using the thunk for now */
+ jmp 20b
+ /* mov \var(%rip), \dst */
+31:
+ .skip -(((11b - 10b)-(31b - 30b)) > 0)*((11b - 10b)-(31b - 30b)), 0x90
+32:
+
+ .pushsection .rai_data, "a"
+40:
+ rai_entry \type 30b 32b 10b 11b 20b
+ .quad \var /* .load.addr */
+41:
+ rai_entry_pad 40b 41b
+ .popsection
+.endm /* rai_load */
+
+
#endif
diff --git a/arch/x86/include/asm/rai.h b/arch/x86/include/asm/rai.h
index 269d696255b0..b57494c98d0f 100644
--- a/arch/x86/include/asm/rai.h
+++ b/arch/x86/include/asm/rai.h
@@ -1,7 +1,10 @@
#ifndef _ASM_X86_RAI_H
#define _ASM_X86_RAI_H
-#define STRUCT_RAI_ENTRY_SIZE 24
+#define RAI_LOAD_4 0
+#define RAI_LOAD_8 1
+
+#define STRUCT_RAI_ENTRY_SIZE 32
/* Put the asm macros in a separate file for easier editing. */
#include <asm/rai.S>
@@ -16,10 +19,35 @@ struct rai_entry {
s32 templ_len; /* length of template */
s32 thunk_offset; /* member-relative offset to ool thunk */
/* type-specific data follows */
+ union {
+ struct {
+ void *addr;
+ } load;
+ };
};
_Static_assert(sizeof(struct rai_entry) == STRUCT_RAI_ENTRY_SIZE,
"please update STRUCT_RAI_ENTRY_SIZE");
+#define _rai_load(var) ({ \
+ typeof(var) ret__; \
+ switch(sizeof(var)) { \
+ case 4: \
+ asm("rai_load %0, %c1, %c2" \
+ : "=r" (ret__) \
+ : "i" (&(var)), "i" (RAI_LOAD_4)); \
+ break; \
+ case 8: \
+ asm("rai_load %0, %c1, %c2" \
+ : "=r" (ret__) \
+ : "i" (&(var)), "i" (RAI_LOAD_8)); \
+ break; \
+ default: \
+ ret__ = _rai_load_fallback(var); \
+ break; \
+ } \
+ ret__; \
+ })
+
#endif /* !__ASSEMBLY */
#endif /* _ASM_X86_RAI_H */
diff --git a/arch/x86/kernel/rai.c b/arch/x86/kernel/rai.c
index 819d03a025e3..e55e85f11a2e 100644
--- a/arch/x86/kernel/rai.c
+++ b/arch/x86/kernel/rai.c
@@ -14,6 +14,24 @@ rai_patch_one(const struct rai_entry *r)
u8 *thunk = (u8*)&r->thunk_offset + r->thunk_offset;
switch (r->type) {
+ case RAI_LOAD_4: {
+ const u32 *imm = r->load.addr;
+ /*
+ * The immediate is the last 4 bytes of the template,
+ * regardless of the operand encoding.
+ */
+ memcpy(templ + r->templ_len - sizeof(*imm), imm, sizeof(*imm));
+ break;
+ }
+ case RAI_LOAD_8: {
+ const u64 *imm = r->load.addr;
+ /*
+ * The immediate is the last 8 bytes of the template,
+ * regardless of the operand encoding.
+ */
+ memcpy(templ + r->templ_len - sizeof(*imm), imm, sizeof(*imm));
+ break;
+ }
default:
WARN_ONCE(1, "unhandled RAI type %d\n", r->type);
return;
--
2.19.1.6.gbde171bbf5