bpf, x32: Fix bug with ALU64 {LSH, RSH, ARSH} BPF_K shift by 0

The current x32 BPF JIT does not correctly compile shift operations when
the immediate shift amount is 0. The expected behavior is for this to
be a no-op.

The following program demonstrates the bug. The expexceted result is 1,
but the current JITed code returns 2.

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

This patch simplifies the code and fixes the bug.

Fixes: 03f5781be2c7 ("bpf, x86_32: add eBPF JIT compiler for ia32")
Co-developed-by: Xi Wang <[e-mail]>
Signed-off-by: Xi Wang <[e-mail]>
Signed-off-by: Luke Nelson <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

arch/x86/net/bpf_jit_comp32.c

···
 	}
 	/* Do LSH operation */
 	if (val < 32) {
-		/* shl dreg_hi,imm8 */
-		EMIT3(0xC1, add_1reg(0xE0, dreg_hi), val);
-		/* mov ebx,dreg_lo */
-		EMIT2(0x8B, add_2reg(0xC0, dreg_lo, IA32_EBX));
+		/* shld dreg_hi,dreg_lo,imm8 */
+		EMIT4(0x0F, 0xA4, add_2reg(0xC0, dreg_hi, dreg_lo), val);
 		/* shl dreg_lo,imm8 */
 		EMIT3(0xC1, add_1reg(0xE0, dreg_lo), val);
-
-		/* IA32_ECX = 32 - val */
-		/* mov ecx,val */
-		EMIT2(0xB1, val);
-		/* movzx ecx,ecx */
-		EMIT3(0x0F, 0xB6, add_2reg(0xC0, IA32_ECX, IA32_ECX));
-		/* 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));
 	} else if (val >= 32 && val < 64) {
 		u32 value = val - 32;
 
···
 
 	/* Do RSH operation */
 	if (val < 32) {
-		/* shr dreg_lo,imm8 */
-		EMIT3(0xC1, add_1reg(0xE8, dreg_lo), val);
-		/* mov ebx,dreg_hi */
-		EMIT2(0x8B, add_2reg(0xC0, dreg_hi, IA32_EBX));
+		/* shrd dreg_lo,dreg_hi,imm8 */
+		EMIT4(0x0F, 0xAC, add_2reg(0xC0, dreg_lo, dreg_hi), val);
 		/* shr dreg_hi,imm8 */
 		EMIT3(0xC1, add_1reg(0xE8, dreg_hi), val);
-
-		/* IA32_ECX = 32 - val */
-		/* mov ecx,val */
-		EMIT2(0xB1, val);
-		/* movzx ecx,ecx */
-		EMIT3(0x0F, 0xB6, add_2reg(0xC0, IA32_ECX, IA32_ECX));
-		/* 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));
 	} else if (val >= 32 && val < 64) {
 		u32 value = val - 32;
 
···
 	}
 	/* Do RSH operation */
 	if (val < 32) {
-		/* shr dreg_lo,imm8 */
-		EMIT3(0xC1, add_1reg(0xE8, dreg_lo), val);
-		/* mov ebx,dreg_hi */
-		EMIT2(0x8B, add_2reg(0xC0, dreg_hi, IA32_EBX));
+		/* shrd dreg_lo,dreg_hi,imm8 */
+		EMIT4(0x0F, 0xAC, add_2reg(0xC0, dreg_lo, dreg_hi), val);
 		/* ashr dreg_hi,imm8 */
 		EMIT3(0xC1, add_1reg(0xF8, dreg_hi), val);
-
-		/* IA32_ECX = 32 - val */
-		/* mov ecx,val */
-		EMIT2(0xB1, val);
-		/* movzx ecx,ecx */
-		EMIT3(0x0F, 0xB6, add_2reg(0xC0, IA32_ECX, IA32_ECX));
-		/* 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));
 	} else if (val >= 32 && val < 64) {
 		u32 value = val - 32;