[PATCH 5.10 059/130] bpf,x86: Simplify computing label offsets

Greg Kroah-Hartman
2022-07-12T18:47:39Z

commit dceba0817ca329868a15e2e1dd46eb6340b69206 upstream.

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
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
[bwh: Backported to 5.10: keep the cnt variable in
emit_bpf_tail_call_{,in}direct()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/net/bpf_jit_comp.c | 125 ++++++++++++++------------------------------
1 file changed, 42 insertions(+), 83 deletions(-)

--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -212,6 +212,14 @@ static void jit_fill_hole(void *area, un

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 */
@@ -371,22 +379,6 @@ int bpf_arch_text_poke(void *ip, enum bp
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:
*
@@ -402,30 +394,12 @@ static int get_pop_bytes(bool *callee_re
* 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;
- int cnt = 0;
-
- /* 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 cnt = 0, offset;

/*
* rdi - pointer to ctx
@@ -440,8 +414,9 @@ static void emit_bpf_tail_call_indirect(
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)
@@ -449,8 +424,9 @@ static void emit_bpf_tail_call_indirect(
*/
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 */

@@ -463,12 +439,11 @@ static void emit_bpf_tail_call_indirect(
* 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)
@@ -488,39 +463,18 @@ static void emit_bpf_tail_call_indirect(
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;
- int cnt = 0;
-
- /* 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 cnt = 0, offset;

/*
* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
@@ -528,28 +482,30 @@ static void emit_bpf_tail_call_direct(st
*/
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, ideal_nops[NOP_ATOMIC5], X86_PATCH_SIZE);
prog += X86_PATCH_SIZE;
+
/* out: */
+ ctx->tail_call_direct_label = prog - start;

*pprog = prog;
}
@@ -1274,13 +1230,16 @@ xadd: 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 */