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target/i386: introduce flags writeback mechanism

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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.
In the case of BZHI, the code has to be modified slightly to ensure
that decode->cc_src is written, otherwise the new assertions trigger.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2023-10-11 15:26:40 +02:00
parent 4b2baf4a55
commit e7bbb7cb71
6 changed files with 101 additions and 13 deletions
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1
target/i386/cpu.h
View File

@ -1285,6 +1285,7 @@ typedef enum {
CC_OP_NB,
} CCOp;
QEMU_BUILD_BUG_ON(CC_OP_NB >= 128);
typedef struct SegmentCache {
uint32_t selector;

34
target/i386/tcg/decode-new.c.inc
View File

@ -1662,6 +1662,7 @@ static void disas_insn_new(DisasContext *s, CPUState *cpu, int b)
bool first = true;
X86DecodedInsn decode;
X86DecodeFunc decode_func = decode_root;
uint8_t cc_live;
s->has_modrm = false;
@ -1815,6 +1816,7 @@ static void disas_insn_new(DisasContext *s, CPUState *cpu, int b)
}
memset(&decode, 0, sizeof(decode));
decode.cc_op = -1;
decode.b = b;
if (!decode_insn(s, env, decode_func, &decode)) {
goto illegal_op;
@ -1953,6 +1955,38 @@ 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_op != -1) {
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_src2, decode.cc_src2);
}
if (decode.cc_op == CC_OP_DYNAMIC) {
tcg_gen_mov_i32(cpu_cc_op, decode.cc_op_dynamic);
}
set_cc_op(s, decode.cc_op);
cc_live = cc_op_live[decode.cc_op];
} else {
cc_live = 0;
}
if (decode.cc_op != CC_OP_DYNAMIC) {
assert(!decode.cc_op_dynamic);
assert(!!decode.cc_dst == !!(cc_live & USES_CC_DST));
assert(!!decode.cc_src == !!(cc_live & USES_CC_SRC));
assert(!!decode.cc_src2 == !!(cc_live & USES_CC_SRC2));
}
return;
gp_fault:
gen_exception_gpf(s);

4
target/i386/tcg/decode-new.h
View File

@ -283,6 +283,10 @@ struct X86DecodedInsn {
target_ulong immediate;
AddressParts mem;
TCGv cc_dst, cc_src, cc_src2;
TCGv_i32 cc_op_dynamic;
int8_t cc_op;
uint8_t b;
};

36
target/i386/tcg/emit.c.inc
View File

@ -339,6 +339,19 @@ 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;
decode->cc_op = op;
}
static void prepare_update2_cc(X86DecodedInsn *decode, DisasContext *s, CCOp op)
{
decode->cc_src = s->T1;
decode->cc_dst = s->T0;
decode->cc_op = op;
}
static void gen_store_sse(DisasContext *s, X86DecodedInsn *decode, int src_ofs)
{
MemOp ot = decode->op[0].ot;
@ -1027,6 +1040,7 @@ static void gen_##uname(DisasContext *s, CPUX86State *env, X86DecodedInsn *decod
VSIB_AVX(VPGATHERD, vpgatherd)
VSIB_AVX(VPGATHERQ, vpgatherq)
/* ADCX/ADOX do not have memory operands and can use set_cc_op. */
static void gen_ADCOX(DisasContext *s, CPUX86State *env, MemOp ot, int cc_op)
{
int opposite_cc_op;
@ -1089,8 +1103,7 @@ 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)
@ -1118,10 +1131,10 @@ 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);
}
/* BLSI do not have memory operands and can use set_cc_op. */
static void gen_BLSI(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
{
MemOp ot = decode->op[0].ot;
@ -1133,6 +1146,7 @@ static void gen_BLSI(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
set_cc_op(s, CC_OP_BMILGB + ot);
}
/* BLSMSK do not have memory operands and can use set_cc_op. */
static void gen_BLSMSK(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
{
MemOp ot = decode->op[0].ot;
@ -1144,6 +1158,7 @@ static void gen_BLSMSK(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode
set_cc_op(s, CC_OP_BMILGB + ot);
}
/* BLSR do not have memory operands and can use set_cc_op. */
static void gen_BLSR(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
{
MemOp ot = decode->op[0].ot;
@ -1164,18 +1179,15 @@ static void gen_BZHI(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
tcg_gen_ext8u_tl(s->T1, s->T1);
tcg_gen_shl_tl(s->A0, mone, s->T1);
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);
/*
* Note that since we're using BMILG (in order to get O
* cleared) we need to store the inverse into C.
*/
tcg_gen_setcond_tl(TCG_COND_LEU, cpu_cc_src, s->T1, bound);
tcg_gen_shl_tl(s->A0, mone, s->T1);
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);
tcg_gen_setcond_tl(TCG_COND_LEU, s->T1, s->T1, bound);
prepare_update2_cc(decode, s, CC_OP_BMILGB + ot);
}
static void gen_CRC32(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)

2
tests/tcg/i386/Makefile.target
View File

@ -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

37
tests/tcg/i386/test-flags.c Normal file
View File

@ -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);
}