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bpf jit: Let the x86 jit handle negative offsets
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Now the helper function from filter.c for negative offsets is exported,
it can be used it in the jit to handle negative offsets.

First modify the asm load helper functions to handle:
- know positive offsets
- know negative offsets
- any offset

then the compiler can be modified to explicitly use these helper
when appropriate.

This fixes the case of a negative X register and allows to lift
the restriction that bpf programs with negative offsets can't
be jited.

Signed-of-by: Jan Seiffert <kaffeemonster@googlemail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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Jan Seiffert authored and David S. Miller committed Apr 3, 2012
1 parent f03fb3f commit a998d43
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Showing 2 changed files with 115 additions and 48 deletions.
122 changes: 91 additions & 31 deletions arch/x86/net/bpf_jit.S
View file
Original file line number Diff line number Diff line change
Expand Up @@ -18,17 +18,17 @@
* r9d : hlen = skb->len - skb->data_len
*/
#define SKBDATA %r8

sk_load_word_ind:
.globl sk_load_word_ind

add %ebx,%esi /* offset += X */
# test %esi,%esi /* if (offset < 0) goto bpf_error; */
js bpf_error
#define SKF_MAX_NEG_OFF $(-0x200000) /* SKF_LL_OFF from filter.h */

sk_load_word:
.globl sk_load_word

test %esi,%esi
js bpf_slow_path_word_neg

sk_load_word_positive_offset:
.globl sk_load_word_positive_offset

mov %r9d,%eax # hlen
sub %esi,%eax # hlen - offset
cmp $3,%eax
Expand All @@ -37,16 +37,15 @@ sk_load_word:
bswap %eax /* ntohl() */
ret


sk_load_half_ind:
.globl sk_load_half_ind

add %ebx,%esi /* offset += X */
js bpf_error

sk_load_half:
.globl sk_load_half

test %esi,%esi
js bpf_slow_path_half_neg

sk_load_half_positive_offset:
.globl sk_load_half_positive_offset

mov %r9d,%eax
sub %esi,%eax # hlen - offset
cmp $1,%eax
Expand All @@ -55,14 +54,15 @@ sk_load_half:
rol $8,%ax # ntohs()
ret

sk_load_byte_ind:
.globl sk_load_byte_ind
add %ebx,%esi /* offset += X */
js bpf_error

sk_load_byte:
.globl sk_load_byte

test %esi,%esi
js bpf_slow_path_byte_neg

sk_load_byte_positive_offset:
.globl sk_load_byte_positive_offset

cmp %esi,%r9d /* if (offset >= hlen) goto bpf_slow_path_byte */
jle bpf_slow_path_byte
movzbl (SKBDATA,%rsi),%eax
Expand All @@ -73,25 +73,21 @@ sk_load_byte:
*
* Implements BPF_S_LDX_B_MSH : ldxb 4*([offset]&0xf)
* Must preserve A accumulator (%eax)
* Inputs : %esi is the offset value, already known positive
* Inputs : %esi is the offset value
*/
ENTRY(sk_load_byte_msh)
CFI_STARTPROC
sk_load_byte_msh:
.globl sk_load_byte_msh
test %esi,%esi
js bpf_slow_path_byte_msh_neg

sk_load_byte_msh_positive_offset:
.globl sk_load_byte_msh_positive_offset
cmp %esi,%r9d /* if (offset >= hlen) goto bpf_slow_path_byte_msh */
jle bpf_slow_path_byte_msh
movzbl (SKBDATA,%rsi),%ebx
and $15,%bl
shl $2,%bl
ret
CFI_ENDPROC
ENDPROC(sk_load_byte_msh)

bpf_error:
# force a return 0 from jit handler
xor %eax,%eax
mov -8(%rbp),%rbx
leaveq
ret

/* rsi contains offset and can be scratched */
#define bpf_slow_path_common(LEN) \
Expand Down Expand Up @@ -138,3 +134,67 @@ bpf_slow_path_byte_msh:
shl $2,%al
xchg %eax,%ebx
ret

#define sk_negative_common(SIZE) \
push %rdi; /* save skb */ \
push %r9; \
push SKBDATA; \
/* rsi already has offset */ \
mov $SIZE,%ecx; /* size */ \
call bpf_internal_load_pointer_neg_helper; \
test %rax,%rax; \
pop SKBDATA; \
pop %r9; \
pop %rdi; \
jz bpf_error


bpf_slow_path_word_neg:
cmp SKF_MAX_NEG_OFF, %esi /* test range */
jl bpf_error /* offset lower -> error */
sk_load_word_negative_offset:
.globl sk_load_word_negative_offset
sk_negative_common(4)
mov (%rax), %eax
bswap %eax
ret

bpf_slow_path_half_neg:
cmp SKF_MAX_NEG_OFF, %esi
jl bpf_error
sk_load_half_negative_offset:
.globl sk_load_half_negative_offset
sk_negative_common(2)
mov (%rax),%ax
rol $8,%ax
movzwl %ax,%eax
ret

bpf_slow_path_byte_neg:
cmp SKF_MAX_NEG_OFF, %esi
jl bpf_error
sk_load_byte_negative_offset:
.globl sk_load_byte_negative_offset
sk_negative_common(1)
movzbl (%rax), %eax
ret

bpf_slow_path_byte_msh_neg:
cmp SKF_MAX_NEG_OFF, %esi
jl bpf_error
sk_load_byte_msh_negative_offset:
.globl sk_load_byte_msh_negative_offset
xchg %eax,%ebx /* dont lose A , X is about to be scratched */
sk_negative_common(1)
movzbl (%rax),%eax
and $15,%al
shl $2,%al
xchg %eax,%ebx
ret

bpf_error:
# force a return 0 from jit handler
xor %eax,%eax
mov -8(%rbp),%rbx
leaveq
ret
41 changes: 24 additions & 17 deletions arch/x86/net/bpf_jit_comp.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,10 @@ int bpf_jit_enable __read_mostly;
* assembly code in arch/x86/net/bpf_jit.S
*/
extern u8 sk_load_word[], sk_load_half[], sk_load_byte[], sk_load_byte_msh[];
extern u8 sk_load_word_ind[], sk_load_half_ind[], sk_load_byte_ind[];
extern u8 sk_load_word_positive_offset[], sk_load_half_positive_offset[];
extern u8 sk_load_byte_positive_offset[], sk_load_byte_msh_positive_offset[];
extern u8 sk_load_word_negative_offset[], sk_load_half_negative_offset[];
extern u8 sk_load_byte_negative_offset[], sk_load_byte_msh_negative_offset[];

static inline u8 *emit_code(u8 *ptr, u32 bytes, unsigned int len)
{
Expand Down Expand Up @@ -117,6 +120,8 @@ static inline void bpf_flush_icache(void *start, void *end)
set_fs(old_fs);
}

#define CHOOSE_LOAD_FUNC(K, func) \
((int)K < 0 ? ((int)K >= SKF_LL_OFF ? func##_negative_offset : func) : func##_positive_offset)

void bpf_jit_compile(struct sk_filter *fp)
{
Expand Down Expand Up @@ -473,44 +478,46 @@ void bpf_jit_compile(struct sk_filter *fp)
#endif
break;
case BPF_S_LD_W_ABS:
func = sk_load_word;
func = CHOOSE_LOAD_FUNC(K, sk_load_word);
common_load: seen |= SEEN_DATAREF;
if ((int)K < 0) {
/* Abort the JIT because __load_pointer() is needed. */
goto out;
}
t_offset = func - (image + addrs[i]);
EMIT1_off32(0xbe, K); /* mov imm32,%esi */
EMIT1_off32(0xe8, t_offset); /* call */
break;
case BPF_S_LD_H_ABS:
func = sk_load_half;
func = CHOOSE_LOAD_FUNC(K, sk_load_half);
goto common_load;
case BPF_S_LD_B_ABS:
func = sk_load_byte;
func = CHOOSE_LOAD_FUNC(K, sk_load_byte);
goto common_load;
case BPF_S_LDX_B_MSH:
if ((int)K < 0) {
/* Abort the JIT because __load_pointer() is needed. */
goto out;
}
func = CHOOSE_LOAD_FUNC(K, sk_load_byte_msh);
seen |= SEEN_DATAREF | SEEN_XREG;
t_offset = sk_load_byte_msh - (image + addrs[i]);
t_offset = func - (image + addrs[i]);
EMIT1_off32(0xbe, K); /* mov imm32,%esi */
EMIT1_off32(0xe8, t_offset); /* call sk_load_byte_msh */
break;
case BPF_S_LD_W_IND:
func = sk_load_word_ind;
func = sk_load_word;
common_load_ind: seen |= SEEN_DATAREF | SEEN_XREG;
t_offset = func - (image + addrs[i]);
EMIT1_off32(0xbe, K); /* mov imm32,%esi */
if (K) {
if (is_imm8(K)) {
EMIT3(0x8d, 0x73, K); /* lea imm8(%rbx), %esi */
} else {
EMIT2(0x8d, 0xb3); /* lea imm32(%rbx),%esi */
EMIT(K, 4);
}
} else {
EMIT2(0x89,0xde); /* mov %ebx,%esi */
}
EMIT1_off32(0xe8, t_offset); /* call sk_load_xxx_ind */
break;
case BPF_S_LD_H_IND:
func = sk_load_half_ind;
func = sk_load_half;
goto common_load_ind;
case BPF_S_LD_B_IND:
func = sk_load_byte_ind;
func = sk_load_byte;
goto common_load_ind;
case BPF_S_JMP_JA:
t_offset = addrs[i + K] - addrs[i];
Expand Down

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