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bpf,x86: Simplify computing label offsets

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Take an idea from the 32bit JIT, which uses the multi-pass nature of
the JIT to compute the instruction offsets on a prior pass in order to
compute the relative jump offsets on a later pass.

Application to the x86_64 JIT is slightly more involved because the
offsets depend on program variables (such as callee_regs_used and
stack_depth) and hence the computed offsets need to be kept in the
context of the JIT.

This removes, IMO quite fragile, code that hard-codes the offsets and
tries to compute the length of variable parts of it.

Convert both emit_bpf_tail_call_*() functions which have an out: label
at the end. Additionally emit_bpt_tail_call_direct() also has a poke
table entry, for which it computes the offset from the end (and thus
already relies on the previous pass to have computed addrs[i]), also
convert this to be a forward based offset.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20211026120310.552304864@infradead.org
This commit is contained in:
Peter Zijlstra 2021-10-26 14:01:47 +02:00
parent f8a66d608a
commit dceba0817c
1 changed files with 42 additions and 81 deletions
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123
arch/x86/net/bpf_jit_comp.c
View File

@ -224,6 +224,14 @@ static void jit_fill_hole(void *area, unsigned int size)
struct jit_context {
int cleanup_addr; /* Epilogue code offset */
/*
* Program specific offsets of labels in the code; these rely on the
* JIT doing at least 2 passes, recording the position on the first
* pass, only to generate the correct offset on the second pass.
*/
int tail_call_direct_label;
int tail_call_indirect_label;
};
/* Maximum number of bytes emitted while JITing one eBPF insn */
@ -379,22 +387,6 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
return __bpf_arch_text_poke(ip, t, old_addr, new_addr, true);
}
static int get_pop_bytes(bool *callee_regs_used)
{
int bytes = 0;
if (callee_regs_used[3])
bytes += 2;
if (callee_regs_used[2])
bytes += 2;
if (callee_regs_used[1])
bytes += 2;
if (callee_regs_used[0])
bytes += 1;
return bytes;
}
/*
* Generate the following code:
*
@ -410,29 +402,12 @@ static int get_pop_bytes(bool *callee_regs_used)
* out:
*/
static void emit_bpf_tail_call_indirect(u8 **pprog, bool *callee_regs_used,
u32 stack_depth)
u32 stack_depth, u8 *ip,
struct jit_context *ctx)
{
int tcc_off = -4 - round_up(stack_depth, 8);
u8 *prog = *pprog;
int pop_bytes = 0;
int off1 = 42;
int off2 = 31;
int off3 = 9;
/* count the additional bytes used for popping callee regs from stack
* that need to be taken into account for each of the offsets that
* are used for bailing out of the tail call
*/
pop_bytes = get_pop_bytes(callee_regs_used);
off1 += pop_bytes;
off2 += pop_bytes;
off3 += pop_bytes;
if (stack_depth) {
off1 += 7;
off2 += 7;
off3 += 7;
}
u8 *prog = *pprog, *start = *pprog;
int offset;
/*
* rdi - pointer to ctx
@ -447,8 +422,9 @@ static void emit_bpf_tail_call_indirect(u8 **pprog, bool *callee_regs_used,
EMIT2(0x89, 0xD2); /* mov edx, edx */
EMIT3(0x39, 0x56, /* cmp dword ptr [rsi + 16], edx */
offsetof(struct bpf_array, map.max_entries));
#define OFFSET1 (off1 + RETPOLINE_RCX_BPF_JIT_SIZE) /* Number of bytes to jump */
EMIT2(X86_JBE, OFFSET1); /* jbe out */
offset = ctx->tail_call_indirect_label - (prog + 2 - start);
EMIT2(X86_JBE, offset); /* jbe out */
/*
* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
@ -456,8 +432,9 @@ static void emit_bpf_tail_call_indirect(u8 **pprog, bool *callee_regs_used,
*/
EMIT2_off32(0x8B, 0x85, tcc_off); /* mov eax, dword ptr [rbp - tcc_off] */
EMIT3(0x83, 0xF8, MAX_TAIL_CALL_CNT); /* cmp eax, MAX_TAIL_CALL_CNT */
#define OFFSET2 (off2 + RETPOLINE_RCX_BPF_JIT_SIZE)
EMIT2(X86_JA, OFFSET2); /* ja out */
offset = ctx->tail_call_indirect_label - (prog + 2 - start);
EMIT2(X86_JA, offset); /* ja out */
EMIT3(0x83, 0xC0, 0x01); /* add eax, 1 */
EMIT2_off32(0x89, 0x85, tcc_off); /* mov dword ptr [rbp - tcc_off], eax */
@ -470,12 +447,11 @@ static void emit_bpf_tail_call_indirect(u8 **pprog, bool *callee_regs_used,
* goto out;
*/
EMIT3(0x48, 0x85, 0xC9); /* test rcx,rcx */
#define OFFSET3 (off3 + RETPOLINE_RCX_BPF_JIT_SIZE)
EMIT2(X86_JE, OFFSET3); /* je out */
*pprog = prog;
pop_callee_regs(pprog, callee_regs_used);
prog = *pprog;
offset = ctx->tail_call_indirect_label - (prog + 2 - start);
EMIT2(X86_JE, offset); /* je out */
pop_callee_regs(&prog, callee_regs_used);
EMIT1(0x58); /* pop rax */
if (stack_depth)
@ -495,38 +471,18 @@ static void emit_bpf_tail_call_indirect(u8 **pprog, bool *callee_regs_used,
RETPOLINE_RCX_BPF_JIT();
/* out: */
ctx->tail_call_indirect_label = prog - start;
*pprog = prog;
}
static void emit_bpf_tail_call_direct(struct bpf_jit_poke_descriptor *poke,
u8 **pprog, int addr, u8 *image,
bool *callee_regs_used, u32 stack_depth)
u8 **pprog, u8 *ip,
bool *callee_regs_used, u32 stack_depth,
struct jit_context *ctx)
{
int tcc_off = -4 - round_up(stack_depth, 8);
u8 *prog = *pprog;
int pop_bytes = 0;
int off1 = 20;
int poke_off;
/* count the additional bytes used for popping callee regs to stack
* that need to be taken into account for jump offset that is used for
* bailing out from of the tail call when limit is reached
*/
pop_bytes = get_pop_bytes(callee_regs_used);
off1 += pop_bytes;
/*
* total bytes for:
* - nop5/ jmpq $off
* - pop callee regs
* - sub rsp, $val if depth > 0
* - pop rax
*/
poke_off = X86_PATCH_SIZE + pop_bytes + 1;
if (stack_depth) {
poke_off += 7;
off1 += 7;
}
u8 *prog = *pprog, *start = *pprog;
int offset;
/*
* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
@ -534,28 +490,30 @@ static void emit_bpf_tail_call_direct(struct bpf_jit_poke_descriptor *poke,
*/
EMIT2_off32(0x8B, 0x85, tcc_off); /* mov eax, dword ptr [rbp - tcc_off] */
EMIT3(0x83, 0xF8, MAX_TAIL_CALL_CNT); /* cmp eax, MAX_TAIL_CALL_CNT */
EMIT2(X86_JA, off1); /* ja out */
offset = ctx->tail_call_direct_label - (prog + 2 - start);
EMIT2(X86_JA, offset); /* ja out */
EMIT3(0x83, 0xC0, 0x01); /* add eax, 1 */
EMIT2_off32(0x89, 0x85, tcc_off); /* mov dword ptr [rbp - tcc_off], eax */
poke->tailcall_bypass = image + (addr - poke_off - X86_PATCH_SIZE);
poke->tailcall_bypass = ip + (prog - start);
poke->adj_off = X86_TAIL_CALL_OFFSET;
poke->tailcall_target = image + (addr - X86_PATCH_SIZE);
poke->tailcall_target = ip + ctx->tail_call_direct_label - X86_PATCH_SIZE;
poke->bypass_addr = (u8 *)poke->tailcall_target + X86_PATCH_SIZE;
emit_jump(&prog, (u8 *)poke->tailcall_target + X86_PATCH_SIZE,
poke->tailcall_bypass);
*pprog = prog;
pop_callee_regs(pprog, callee_regs_used);
prog = *pprog;
pop_callee_regs(&prog, callee_regs_used);
EMIT1(0x58); /* pop rax */
if (stack_depth)
EMIT3_off32(0x48, 0x81, 0xC4, round_up(stack_depth, 8));
memcpy(prog, x86_nops[5], X86_PATCH_SIZE);
prog += X86_PATCH_SIZE;
/* out: */
ctx->tail_call_direct_label = prog - start;
*pprog = prog;
}
@ -1411,13 +1369,16 @@ st: if (is_imm8(insn->off))
case BPF_JMP | BPF_TAIL_CALL:
if (imm32)
emit_bpf_tail_call_direct(&bpf_prog->aux->poke_tab[imm32 - 1],
&prog, addrs[i], image,
&prog, image + addrs[i - 1],
callee_regs_used,
bpf_prog->aux->stack_depth);
bpf_prog->aux->stack_depth,
ctx);
else
emit_bpf_tail_call_indirect(&prog,
callee_regs_used,
bpf_prog->aux->stack_depth);
bpf_prog->aux->stack_depth,
image + addrs[i - 1],
ctx);
break;
/* cond jump */