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Subject[PATCH bpf 1/3] bpf, x32: Fix bug with ALU64 {LSH,RSH,ARSH} BPF_X shift by 0
Date
The current x32 BPF JIT for shift operations is not correct when the
shift amount in a register is 0. The expected behavior is a no-op, whereas
the current implementation changes bits in the destination register.

The following example demonstrates the bug. The expected result of this
program is 1, but the current JITed code returns 2.

r0 = 1
r1 = 1
r2 = 0
r1 <<= r2
if r1 == 1 goto end
r0 = 2
end:
exit

The bug is caused by an incorrect assumption by the JIT that a shift by
32 clear the register. On x32 however, shifts use the lower 5 bits of
the source, making a shift by 32 equivalent to a shift by 0.

This patch fixes the bug using double-precision shifts, which also
simplifies the code.

Fixes: 03f5781be2c7 ("bpf, x86_32: add eBPF JIT compiler for ia32")
Co-developed-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
---
arch/x86/net/bpf_jit_comp32.c | 221 ++++------------------------------
1 file changed, 23 insertions(+), 198 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp32.c b/arch/x86/net/bpf_jit_comp32.c
index b29e82f190c7..f34ef513f4f9 100644
--- a/arch/x86/net/bpf_jit_comp32.c
+++ b/arch/x86/net/bpf_jit_comp32.c
@@ -724,9 +724,6 @@ static inline void emit_ia32_lsh_r64(const u8 dst[], const u8 src[],
{
u8 *prog = *pprog;
int cnt = 0;
- static int jmp_label1 = -1;
- static int jmp_label2 = -1;
- static int jmp_label3 = -1;
u8 dreg_lo = dstk ? IA32_EAX : dst_lo;
u8 dreg_hi = dstk ? IA32_EDX : dst_hi;

@@ -745,79 +742,23 @@ static inline void emit_ia32_lsh_r64(const u8 dst[], const u8 src[],
/* mov ecx,src_lo */
EMIT2(0x8B, add_2reg(0xC0, src_lo, IA32_ECX));

- /* cmp ecx,32 */
- EMIT3(0x83, add_1reg(0xF8, IA32_ECX), 32);
- /* Jumps when >= 32 */
- if (is_imm8(jmp_label(jmp_label1, 2)))
- EMIT2(IA32_JAE, jmp_label(jmp_label1, 2));
- else
- EMIT2_off32(0x0F, IA32_JAE + 0x10, jmp_label(jmp_label1, 6));
-
- /* < 32 */
- /* shl dreg_hi,cl */
- EMIT2(0xD3, add_1reg(0xE0, dreg_hi));
- /* mov ebx,dreg_lo */
- EMIT2(0x8B, add_2reg(0xC0, dreg_lo, IA32_EBX));
+ /* shld dreg_hi,dreg_lo,cl */
+ EMIT3(0x0F, 0xA5, add_2reg(0xC0, dreg_hi, dreg_lo));
/* shl dreg_lo,cl */
EMIT2(0xD3, add_1reg(0xE0, dreg_lo));

- /* IA32_ECX = -IA32_ECX + 32 */
- /* neg ecx */
- EMIT2(0xF7, add_1reg(0xD8, IA32_ECX));
- /* add ecx,32 */
- EMIT3(0x83, add_1reg(0xC0, IA32_ECX), 32);
-
- /* shr ebx,cl */
- EMIT2(0xD3, add_1reg(0xE8, IA32_EBX));
- /* or dreg_hi,ebx */
- EMIT2(0x09, add_2reg(0xC0, dreg_hi, IA32_EBX));
-
- /* goto out; */
- if (is_imm8(jmp_label(jmp_label3, 2)))
- EMIT2(0xEB, jmp_label(jmp_label3, 2));
- else
- EMIT1_off32(0xE9, jmp_label(jmp_label3, 5));
-
- /* >= 32 */
- if (jmp_label1 == -1)
- jmp_label1 = cnt;
+ /* if ecx >= 32, mov dreg_lo into dreg_hi and clear dreg_lo */

- /* cmp ecx,64 */
- EMIT3(0x83, add_1reg(0xF8, IA32_ECX), 64);
- /* Jumps when >= 64 */
- if (is_imm8(jmp_label(jmp_label2, 2)))
- EMIT2(IA32_JAE, jmp_label(jmp_label2, 2));
- else
- EMIT2_off32(0x0F, IA32_JAE + 0x10, jmp_label(jmp_label2, 6));
+ /* cmp ecx,32 */
+ EMIT3(0x83, add_1reg(0xF8, IA32_ECX), 32);
+ /* skip the next two instructions (4 bytes) when < 32 */
+ EMIT2(IA32_JB, 4);

- /* >= 32 && < 64 */
- /* sub ecx,32 */
- EMIT3(0x83, add_1reg(0xE8, IA32_ECX), 32);
- /* shl dreg_lo,cl */
- EMIT2(0xD3, add_1reg(0xE0, dreg_lo));
/* mov dreg_hi,dreg_lo */
EMIT2(0x89, add_2reg(0xC0, dreg_hi, dreg_lo));
-
/* xor dreg_lo,dreg_lo */
EMIT2(0x33, add_2reg(0xC0, dreg_lo, dreg_lo));

- /* goto out; */
- if (is_imm8(jmp_label(jmp_label3, 2)))
- EMIT2(0xEB, jmp_label(jmp_label3, 2));
- else
- EMIT1_off32(0xE9, jmp_label(jmp_label3, 5));
-
- /* >= 64 */
- if (jmp_label2 == -1)
- jmp_label2 = cnt;
- /* xor dreg_lo,dreg_lo */
- EMIT2(0x33, add_2reg(0xC0, dreg_lo, dreg_lo));
- /* xor dreg_hi,dreg_hi */
- EMIT2(0x33, add_2reg(0xC0, dreg_hi, dreg_hi));
-
- if (jmp_label3 == -1)
- jmp_label3 = cnt;
-
if (dstk) {
/* mov dword ptr [ebp+off],dreg_lo */
EMIT3(0x89, add_2reg(0x40, IA32_EBP, dreg_lo),
@@ -836,9 +777,6 @@ static inline void emit_ia32_arsh_r64(const u8 dst[], const u8 src[],
{
u8 *prog = *pprog;
int cnt = 0;
- static int jmp_label1 = -1;
- static int jmp_label2 = -1;
- static int jmp_label3 = -1;
u8 dreg_lo = dstk ? IA32_EAX : dst_lo;
u8 dreg_hi = dstk ? IA32_EDX : dst_hi;

@@ -857,78 +795,22 @@ static inline void emit_ia32_arsh_r64(const u8 dst[], const u8 src[],
/* mov ecx,src_lo */
EMIT2(0x8B, add_2reg(0xC0, src_lo, IA32_ECX));

- /* cmp ecx,32 */
- EMIT3(0x83, add_1reg(0xF8, IA32_ECX), 32);
- /* Jumps when >= 32 */
- if (is_imm8(jmp_label(jmp_label1, 2)))
- EMIT2(IA32_JAE, jmp_label(jmp_label1, 2));
- else
- EMIT2_off32(0x0F, IA32_JAE + 0x10, jmp_label(jmp_label1, 6));
-
- /* < 32 */
- /* lshr dreg_lo,cl */
- EMIT2(0xD3, add_1reg(0xE8, dreg_lo));
- /* mov ebx,dreg_hi */
- EMIT2(0x8B, add_2reg(0xC0, dreg_hi, IA32_EBX));
- /* ashr dreg_hi,cl */
+ /* shrd dreg_lo,dreg_hi,cl */
+ EMIT3(0x0F, 0xAD, add_2reg(0xC0, dreg_lo, dreg_hi));
+ /* sar dreg_hi,cl */
EMIT2(0xD3, add_1reg(0xF8, dreg_hi));

- /* IA32_ECX = -IA32_ECX + 32 */
- /* neg ecx */
- EMIT2(0xF7, add_1reg(0xD8, IA32_ECX));
- /* add ecx,32 */
- EMIT3(0x83, add_1reg(0xC0, IA32_ECX), 32);
-
- /* shl ebx,cl */
- EMIT2(0xD3, add_1reg(0xE0, IA32_EBX));
- /* or dreg_lo,ebx */
- EMIT2(0x09, add_2reg(0xC0, dreg_lo, IA32_EBX));
-
- /* goto out; */
- if (is_imm8(jmp_label(jmp_label3, 2)))
- EMIT2(0xEB, jmp_label(jmp_label3, 2));
- else
- EMIT1_off32(0xE9, jmp_label(jmp_label3, 5));
-
- /* >= 32 */
- if (jmp_label1 == -1)
- jmp_label1 = cnt;
+ /* if ecx >= 32, mov dreg_hi to dreg_lo and set/clear dreg_hi depending on sign */

- /* cmp ecx,64 */
- EMIT3(0x83, add_1reg(0xF8, IA32_ECX), 64);
- /* Jumps when >= 64 */
- if (is_imm8(jmp_label(jmp_label2, 2)))
- EMIT2(IA32_JAE, jmp_label(jmp_label2, 2));
- else
- EMIT2_off32(0x0F, IA32_JAE + 0x10, jmp_label(jmp_label2, 6));
+ /* cmp ecx,32 */
+ EMIT3(0x83, add_1reg(0xF8, IA32_ECX), 32);
+ /* skip the next two instructions (5 bytes) when < 32 */
+ EMIT2(IA32_JB, 5);

- /* >= 32 && < 64 */
- /* sub ecx,32 */
- EMIT3(0x83, add_1reg(0xE8, IA32_ECX), 32);
- /* ashr dreg_hi,cl */
- EMIT2(0xD3, add_1reg(0xF8, dreg_hi));
/* mov dreg_lo,dreg_hi */
EMIT2(0x89, add_2reg(0xC0, dreg_lo, dreg_hi));
-
- /* ashr dreg_hi,imm8 */
- EMIT3(0xC1, add_1reg(0xF8, dreg_hi), 31);
-
- /* goto out; */
- if (is_imm8(jmp_label(jmp_label3, 2)))
- EMIT2(0xEB, jmp_label(jmp_label3, 2));
- else
- EMIT1_off32(0xE9, jmp_label(jmp_label3, 5));
-
- /* >= 64 */
- if (jmp_label2 == -1)
- jmp_label2 = cnt;
- /* ashr dreg_hi,imm8 */
+ /* sar dreg_hi,31 */
EMIT3(0xC1, add_1reg(0xF8, dreg_hi), 31);
- /* mov dreg_lo,dreg_hi */
- EMIT2(0x89, add_2reg(0xC0, dreg_lo, dreg_hi));
-
- if (jmp_label3 == -1)
- jmp_label3 = cnt;

if (dstk) {
/* mov dword ptr [ebp+off],dreg_lo */
@@ -948,9 +830,6 @@ static inline void emit_ia32_rsh_r64(const u8 dst[], const u8 src[], bool dstk,
{
u8 *prog = *pprog;
int cnt = 0;
- static int jmp_label1 = -1;
- static int jmp_label2 = -1;
- static int jmp_label3 = -1;
u8 dreg_lo = dstk ? IA32_EAX : dst_lo;
u8 dreg_hi = dstk ? IA32_EDX : dst_hi;

@@ -969,77 +848,23 @@ static inline void emit_ia32_rsh_r64(const u8 dst[], const u8 src[], bool dstk,
/* mov ecx,src_lo */
EMIT2(0x8B, add_2reg(0xC0, src_lo, IA32_ECX));

- /* cmp ecx,32 */
- EMIT3(0x83, add_1reg(0xF8, IA32_ECX), 32);
- /* Jumps when >= 32 */
- if (is_imm8(jmp_label(jmp_label1, 2)))
- EMIT2(IA32_JAE, jmp_label(jmp_label1, 2));
- else
- EMIT2_off32(0x0F, IA32_JAE + 0x10, jmp_label(jmp_label1, 6));
-
- /* < 32 */
- /* lshr dreg_lo,cl */
- EMIT2(0xD3, add_1reg(0xE8, dreg_lo));
- /* mov ebx,dreg_hi */
- EMIT2(0x8B, add_2reg(0xC0, dreg_hi, IA32_EBX));
+ /* shrd dreg_lo,dreg_hi,cl */
+ EMIT3(0x0F, 0xAD, add_2reg(0xC0, dreg_lo, dreg_hi));
/* shr dreg_hi,cl */
EMIT2(0xD3, add_1reg(0xE8, dreg_hi));

- /* IA32_ECX = -IA32_ECX + 32 */
- /* neg ecx */
- EMIT2(0xF7, add_1reg(0xD8, IA32_ECX));
- /* add ecx,32 */
- EMIT3(0x83, add_1reg(0xC0, IA32_ECX), 32);
-
- /* shl ebx,cl */
- EMIT2(0xD3, add_1reg(0xE0, IA32_EBX));
- /* or dreg_lo,ebx */
- EMIT2(0x09, add_2reg(0xC0, dreg_lo, IA32_EBX));
+ /* if ecx >= 32, mov dreg_hi to dreg_lo and clear dreg_hi */

- /* goto out; */
- if (is_imm8(jmp_label(jmp_label3, 2)))
- EMIT2(0xEB, jmp_label(jmp_label3, 2));
- else
- EMIT1_off32(0xE9, jmp_label(jmp_label3, 5));
-
- /* >= 32 */
- if (jmp_label1 == -1)
- jmp_label1 = cnt;
- /* cmp ecx,64 */
- EMIT3(0x83, add_1reg(0xF8, IA32_ECX), 64);
- /* Jumps when >= 64 */
- if (is_imm8(jmp_label(jmp_label2, 2)))
- EMIT2(IA32_JAE, jmp_label(jmp_label2, 2));
- else
- EMIT2_off32(0x0F, IA32_JAE + 0x10, jmp_label(jmp_label2, 6));
+ /* cmp ecx,32 */
+ EMIT3(0x83, add_1reg(0xF8, IA32_ECX), 32);
+ /* skip the next two instructions (4 bytes) when < 32 */
+ EMIT2(IA32_JB, 4);

- /* >= 32 && < 64 */
- /* sub ecx,32 */
- EMIT3(0x83, add_1reg(0xE8, IA32_ECX), 32);
- /* shr dreg_hi,cl */
- EMIT2(0xD3, add_1reg(0xE8, dreg_hi));
/* mov dreg_lo,dreg_hi */
EMIT2(0x89, add_2reg(0xC0, dreg_lo, dreg_hi));
/* xor dreg_hi,dreg_hi */
EMIT2(0x33, add_2reg(0xC0, dreg_hi, dreg_hi));

- /* goto out; */
- if (is_imm8(jmp_label(jmp_label3, 2)))
- EMIT2(0xEB, jmp_label(jmp_label3, 2));
- else
- EMIT1_off32(0xE9, jmp_label(jmp_label3, 5));
-
- /* >= 64 */
- if (jmp_label2 == -1)
- jmp_label2 = cnt;
- /* xor dreg_lo,dreg_lo */
- EMIT2(0x33, add_2reg(0xC0, dreg_lo, dreg_lo));
- /* xor dreg_hi,dreg_hi */
- EMIT2(0x33, add_2reg(0xC0, dreg_hi, dreg_hi));
-
- if (jmp_label3 == -1)
- jmp_label3 = cnt;
-
if (dstk) {
/* mov dword ptr [ebp+off],dreg_lo */
EMIT3(0x89, add_2reg(0x40, IA32_EBP, dreg_lo),
--
2.20.1
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