[PATCH 07/18] target/i386: introduce flags writeback mechanism

[Paolo Bonzini <pbonzini@redhat.com>]

ALU instructions can write to both memory and flags.  If the CC_SRC*
and CC_DST locations have been written already when a memory access
causes a fault, the value in CC_SRC* and CC_DST might be interpreted
with the wrong CC_OP (the one that is in effect before the instruction.

Besides just using the wrong result for the flags, something like
subtracting -1 can have disastrous effects if the current CC_OP is
CC_OP_EFLAGS: this is because QEMU does not expect bits outside the ALU
flags to be set in CC_SRC, and env->eflags can end up set to all-ones.
In the case of the attached testcase, this sets IOPL to 3 and would
cause an assertion failure if SUB is moved to the new decoder.

This mechanism is not really needed for BMI instructions, which can
only write to a register, but put it to use anyway for cleanliness.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/tcg/decode-new.c.inc | 20 +++++++++++++++++
 target/i386/tcg/decode-new.h     |  2 ++
 target/i386/tcg/emit.c.inc       | 15 +++++++------
 tests/tcg/i386/Makefile.target   |  2 +-
 tests/tcg/i386/test-flags.c      | 37 ++++++++++++++++++++++++++++++++
 5 files changed, 69 insertions(+), 7 deletions(-)
 create mode 100644 tests/tcg/i386/test-flags.c

diff --git a/target/i386/tcg/decode-new.c.inc b/target/i386/tcg/decode-new.c.inc
index eb2400095f8..bad561ff66d 100644
--- a/target/i386/tcg/decode-new.c.inc
+++ b/target/i386/tcg/decode-new.c.inc
@@ -1943,6 +1943,26 @@ static void disas_insn_new(DisasContext *s, CPUState *cpu, int b)
         decode.e.gen(s, env, &decode);
         gen_writeback(s, &decode, 0, s->T0);
     }
+
+    /*
+     * Write back flags after last memory access.  Some newer ALU instructions, as
+     * well as SSE instructions, write flags in the gen_* function, but that can
+     * cause incorrect tracking of CC_OP for instructions that write to both memory
+     * and flags.
+     */
+    if (decode.cc_dst) {
+        tcg_gen_mov_tl(cpu_cc_dst, decode.cc_dst);
+    }
+    if (decode.cc_src) {
+        tcg_gen_mov_tl(cpu_cc_src, decode.cc_src);
+    }
+    if (decode.cc_src2) {
+        tcg_gen_mov_tl(cpu_cc_src, decode.cc_src2);
+    }
+    if (decode.cc_srcT) {
+        tcg_gen_mov_tl(s->cc_srcT, decode.cc_srcT);
+    }
+
     return;
  gp_fault:
     gen_exception_gpf(s);
diff --git a/target/i386/tcg/decode-new.h b/target/i386/tcg/decode-new.h
index ab21fa6db97..b2879136614 100644
--- a/target/i386/tcg/decode-new.h
+++ b/target/i386/tcg/decode-new.h
@@ -265,6 +265,8 @@ struct X86DecodedInsn {
     target_ulong immediate;
     AddressParts mem;
 
+    TCGv cc_dst, cc_src, cc_src2, cc_srcT;
+
     uint8_t b;
 };
 
diff --git a/target/i386/tcg/emit.c.inc b/target/i386/tcg/emit.c.inc
index 82da5488d47..b5dfdc409e5 100644
--- a/target/i386/tcg/emit.c.inc
+++ b/target/i386/tcg/emit.c.inc
@@ -323,6 +323,12 @@ static inline int vector_len(DisasContext *s, X86DecodedInsn *decode)
     return s->vex_l ? 32 : 16;
 }
 
+static void prepare_update1_cc(X86DecodedInsn *decode, DisasContext *s, CCOp op)
+{
+    decode->cc_dst = s->T0;
+    set_cc_op(s, op);
+}
+
 static void gen_store_sse(DisasContext *s, X86DecodedInsn *decode, int src_ofs)
 {
     MemOp ot = decode->op[0].ot;
@@ -1073,8 +1079,7 @@ static void gen_ANDN(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
     MemOp ot = decode->op[0].ot;
 
     tcg_gen_andc_tl(s->T0, s->T1, s->T0);
-    gen_op_update1_cc(s);
-    set_cc_op(s, CC_OP_LOGICB + ot);
+    prepare_update1_cc(decode, s, CC_OP_LOGICB + ot);
 }
 
 static void gen_BEXTR(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
@@ -1105,8 +1110,7 @@ static void gen_BEXTR(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
     tcg_gen_movcond_tl(TCG_COND_LEU, s->T1, s->A0, bound, s->T1, zero);
     tcg_gen_andc_tl(s->T0, s->T0, s->T1);
 
-    gen_op_update1_cc(s);
-    set_cc_op(s, CC_OP_LOGICB + ot);
+    prepare_update1_cc(decode, s, CC_OP_LOGICB + ot);
 }
 
 static void gen_BLSI(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
@@ -1161,8 +1165,7 @@ static void gen_BZHI(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
     tcg_gen_movcond_tl(TCG_COND_LEU, s->A0, s->T1, bound, s->A0, zero);
     tcg_gen_andc_tl(s->T0, s->T0, s->A0);
 
-    gen_op_update1_cc(s);
-    set_cc_op(s, CC_OP_BMILGB + ot);
+    prepare_update1_cc(decode, s, CC_OP_BMILGB + ot);
 }
 
 static void gen_CRC32(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
diff --git a/tests/tcg/i386/Makefile.target b/tests/tcg/i386/Makefile.target
index fdf757c6ce4..ca0f543ef16 100644
--- a/tests/tcg/i386/Makefile.target
+++ b/tests/tcg/i386/Makefile.target
@@ -13,7 +13,7 @@ config-cc.mak: Makefile
 
 I386_SRCS=$(notdir $(wildcard $(I386_SRC)/*.c))
 ALL_X86_TESTS=$(I386_SRCS:.c=)
-SKIP_I386_TESTS=test-i386-ssse3 test-avx test-3dnow test-mmx
+SKIP_I386_TESTS=test-i386-ssse3 test-avx test-3dnow test-mmx test-flags
 X86_64_TESTS:=$(filter test-i386-adcox test-i386-bmi2 $(SKIP_I386_TESTS), $(ALL_X86_TESTS))
 
 test-i386-sse-exceptions: CFLAGS += -msse4.1 -mfpmath=sse
diff --git a/tests/tcg/i386/test-flags.c b/tests/tcg/i386/test-flags.c
new file mode 100644
index 00000000000..c379e296275
--- /dev/null
+++ b/tests/tcg/i386/test-flags.c
@@ -0,0 +1,37 @@
+#define _GNU_SOURCE
+#include <sys/mman.h>
+#include <signal.h>
+#include <stdio.h>
+#include <assert.h>
+
+volatile unsigned long flags;
+volatile unsigned long flags_after;
+int *addr;
+
+void sigsegv(int sig, siginfo_t *info, ucontext_t *uc)
+{
+    flags = uc->uc_mcontext.gregs[REG_EFL];
+    mprotect(addr, 4096, PROT_READ|PROT_WRITE);
+}
+
+int main()
+{
+    struct sigaction sa = { .sa_handler = (void *)sigsegv, .sa_flags = SA_SIGINFO };
+    sigaction(SIGSEGV, &sa, NULL);
+
+    /* fault in the page then protect it */
+    addr = mmap (NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);
+    *addr = 0x1234;
+    mprotect(addr, 4096, PROT_READ);
+
+    asm("# set flags to all ones    \n"
+        "mov $-1, %%eax             \n"
+        "movq addr, %%rdi           \n"
+        "sahf                       \n"
+        "sub %%eax, (%%rdi)         \n"
+        "pushf                      \n"
+        "pop  flags_after(%%rip)    \n" : : : "eax", "edi", "memory");
+
+    /* OF can have any value before the SUB instruction.  */
+    assert((flags & 0xff) == 0xd7 && (flags_after & 0x8ff) == 0x17);
+}
-- 
2.41.0