[PATCH v5 bpf-next 1/3] bpf, x64: Fix tailcall hierarchy

Leon Hwang
2024-06-23T16:15:59Z

This patch fixes a tailcall issue caused by abusing the tailcall in
bpf2bpf feature.

As we know, tail_call_cnt propagates by rax from caller to callee when
to call subprog in tailcall context. But, like the following example,
MAX_TAIL_CALL_CNT won't work because of missing tail_call_cnt
back-propagation from callee to caller.

\#include <linux/bpf.h>
\#include <bpf/bpf_helpers.h>
\#include "bpf_legacy.h"

struct {
__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
__uint(max_entries, 1);
__uint(key_size, sizeof(__u32));
__uint(value_size, sizeof(__u32));
} jmp_table SEC(".maps");

int count = 0;

static __noinline
int subprog_tail1(struct __sk_buff *skb)
{
bpf_tail_call_static(skb, &jmp_table, 0);
return 0;
}

static __noinline
int subprog_tail2(struct __sk_buff *skb)
{
bpf_tail_call_static(skb, &jmp_table, 0);
return 0;
}

SEC("tc")
int entry(struct __sk_buff *skb)
{
volatile int ret = 1;

count++;
subprog_tail1(skb);
subprog_tail2(skb);

return ret;
}

char __license[] SEC("license") = "GPL";

At run time, the tail_call_cnt in entry() will be propagated to
subprog_tail1() and subprog_tail2(). But, when the tail_call_cnt in
subprog_tail1() updates when bpf_tail_call_static(), the tail_call_cnt
in entry() won't be updated at the same time. As a result, in entry(),
when tail_call_cnt in entry() is less than MAX_TAIL_CALL_CNT and
subprog_tail1() returns because of MAX_TAIL_CALL_CNT limit,
bpf_tail_call_static() in suprog_tail2() is able to run because the
tail_call_cnt in subprog_tail2() propagated from entry() is less than
MAX_TAIL_CALL_CNT.

So, how many tailcalls are there for this case if no error happens?

From top-down view, does it look like hierarchy layer and layer?

With this view, there will be 2+4+8+...+2^33 = 2^34 - 2 = 17,179,869,182
tailcalls for this case.

How about there are N subprog_tail() in entry()? There will be almost
N^34 tailcalls.

Then, in this patch, it resolves this case on x86_64.

In stead of propagating tail_call_cnt from caller to callee, it
propagates its pointer, tail_call_cnt_ptr, tcc_ptr for short.

However, where does it store tail_call_cnt?

It stores tail_call_cnt on the stack of main prog. When tail call
happens in subprog, it increments tail_call_cnt by tcc_ptr.

Meanwhile, it stores tail_call_cnt_ptr on the stack of main prog, too.

And, before jump to tail callee, it has to pop tail_call_cnt and
tail_call_cnt_ptr.

Then, at the prologue of subprog, it must not make rax as
tail_call_cnt_ptr again. It has to reuse tail_call_cnt_ptr from caller.

As a result, at run time, it has to recognize rax is tail_call_cnt or
tail_call_cnt_ptr at prologue by:

1. rax is tail_call_cnt if rax is <= MAX_TAIL_CALL_CNT.
2. rax is tail_call_cnt_ptr if rax is > MAX_TAIL_CALL_CNT, because a
pointer won't be <= MAX_TAIL_CALL_CNT.

Furthermore, when trampoline is the caller of bpf prog, which is
tail_call_reachable, it is required to propagate rax through trampoline.

Fixes: ebf7d1f508a7 ("bpf, x64: rework pro/epilogue and tailcall handling in JIT")
Fixes: e411901c0b77 ("bpf: allow for tailcalls in BPF subprograms for x64 JIT")
Signed-off-by: Leon Hwang <hffilwlqm@gmail.com>
---
arch/x86/net/bpf_jit_comp.c | 107 ++++++++++++++++++++++++++----------
1 file changed, 79 insertions(+), 28 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index d25d81c8ecc00..074b41fafbe3f 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -273,7 +273,7 @@ struct jit_context {
/* Number of bytes emit_patch() needs to generate instructions */
#define X86_PATCH_SIZE 5
/* Number of bytes that will be skipped on tailcall */
-#define X86_TAIL_CALL_OFFSET (11 + ENDBR_INSN_SIZE)
+#define X86_TAIL_CALL_OFFSET (12 + ENDBR_INSN_SIZE)

static void push_r12(u8 **pprog)
{
@@ -403,6 +403,37 @@ static void emit_cfi(u8 **pprog, u32 hash)
*pprog = prog;
}

+static void emit_prologue_tail_call(u8 **pprog, bool is_subprog)
+{
+ u8 *prog = *pprog;
+
+ if (!is_subprog) {
+ /* cmp rax, MAX_TAIL_CALL_CNT */
+ EMIT4(0x48, 0x83, 0xF8, MAX_TAIL_CALL_CNT);
+ EMIT2(X86_JA, 6); /* ja 6 */
+ /* rax is tail_call_cnt if <= MAX_TAIL_CALL_CNT.
+ * case1: entry of main prog.
+ * case2: tail callee of main prog.
+ */
+ EMIT1(0x50); /* push rax */
+ /* Make rax as tail_call_cnt_ptr. */
+ EMIT3(0x48, 0x89, 0xE0); /* mov rax, rsp */
+ EMIT2(0xEB, 1); /* jmp 1 */
+ /* rax is tail_call_cnt_ptr if > MAX_TAIL_CALL_CNT.
+ * case: tail callee of subprog.
+ */
+ EMIT1(0x50); /* push rax */
+ /* push tail_call_cnt_ptr */
+ EMIT1(0x50); /* push rax */
+ } else { /* is_subprog */
+ /* rax is tail_call_cnt_ptr. */
+ EMIT1(0x50); /* push rax */
+ EMIT1(0x50); /* push rax */
+ }
+
+ *pprog = prog;
+}
+
/*
* Emit x86-64 prologue code for BPF program.
* bpf_tail_call helper will skip the first X86_TAIL_CALL_OFFSET bytes
@@ -424,10 +455,10 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
/* When it's the entry of the whole tailcall context,
* zeroing rax means initialising tail_call_cnt.
*/
- EMIT2(0x31, 0xC0); /* xor eax, eax */
+ EMIT3(0x48, 0x31, 0xC0); /* xor rax, rax */
else
/* Keep the same instruction layout. */
- EMIT2(0x66, 0x90); /* nop2 */
+ emit_nops(&prog, 3); /* nop3 */
}
/* Exception callback receives FP as third parameter */
if (is_exception_cb) {
@@ -453,7 +484,7 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
if (stack_depth)
EMIT3_off32(0x48, 0x81, 0xEC, round_up(stack_depth, 8));
if (tail_call_reachable)
- EMIT1(0x50); /* push rax */
+ emit_prologue_tail_call(&prog, is_subprog);
*pprog = prog;
}

@@ -589,13 +620,15 @@ static void emit_return(u8 **pprog, u8 *ip)
*pprog = prog;
}

+#define BPF_TAIL_CALL_CNT_PTR_STACK_OFF(stack) (-16 - round_up(stack, 8))
+
/*
* Generate the following code:
*
* ... bpf_tail_call(void *ctx, struct bpf_array *array, u64 index) ...
* if (index >= array->map.max_entries)
* goto out;
- * if (tail_call_cnt++ >= MAX_TAIL_CALL_CNT)
+ * if ((*tcc_ptr)++ >= MAX_TAIL_CALL_CNT)
* goto out;
* prog = array->ptrs[index];
* if (prog == NULL)
@@ -608,7 +641,7 @@ static void emit_bpf_tail_call_indirect(struct bpf_prog *bpf_prog,
u32 stack_depth, u8 *ip,
struct jit_context *ctx)
{
- int tcc_off = -4 - round_up(stack_depth, 8);
+ int tcc_ptr_off = BPF_TAIL_CALL_CNT_PTR_STACK_OFF(stack_depth);
u8 *prog = *pprog, *start = *pprog;
int offset;

@@ -630,16 +663,14 @@ static void emit_bpf_tail_call_indirect(struct bpf_prog *bpf_prog,
EMIT2(X86_JBE, offset); /* jbe out */

/*
- * if (tail_call_cnt++ >= MAX_TAIL_CALL_CNT)
+ * if ((*tcc_ptr)++ >= MAX_TAIL_CALL_CNT)
* goto out;
*/
- 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 */
+ EMIT3_off32(0x48, 0x8B, 0x85, tcc_ptr_off); /* mov rax, qword ptr [rbp - tcc_ptr_off] */
+ EMIT4(0x48, 0x83, 0x38, MAX_TAIL_CALL_CNT); /* cmp qword ptr [rax], MAX_TAIL_CALL_CNT */

offset = ctx->tail_call_indirect_label - (prog + 2 - start);
EMIT2(X86_JAE, offset); /* jae out */
- EMIT3(0x83, 0xC0, 0x01); /* add eax, 1 */
- EMIT2_off32(0x89, 0x85, tcc_off); /* mov dword ptr [rbp - tcc_off], eax */

/* prog = array->ptrs[index]; */
EMIT4_off32(0x48, 0x8B, 0x8C, 0xD6, /* mov rcx, [rsi + rdx * 8 + offsetof(...)] */
@@ -654,6 +685,9 @@ static void emit_bpf_tail_call_indirect(struct bpf_prog *bpf_prog,
offset = ctx->tail_call_indirect_label - (prog + 2 - start);
EMIT2(X86_JE, offset); /* je out */

+ /* Inc tail_call_cnt if the slot is populated. */
+ EMIT4(0x48, 0x83, 0x00, 0x01); /* add qword ptr [rax], 1 */
+
if (bpf_prog->aux->exception_boundary) {
pop_callee_regs(&prog, all_callee_regs_used);
pop_r12(&prog);
@@ -663,6 +697,11 @@ static void emit_bpf_tail_call_indirect(struct bpf_prog *bpf_prog,
pop_r12(&prog);
}

+ /* Pop tail_call_cnt_ptr. */
+ EMIT1(0x58); /* pop rax */
+ /* Pop tail_call_cnt, if it's main prog.
+ * Pop tail_call_cnt_ptr, if it's subprog.
+ */
EMIT1(0x58); /* pop rax */
if (stack_depth)
EMIT3_off32(0x48, 0x81, 0xC4, /* add rsp, sd */
@@ -691,21 +730,19 @@ static void emit_bpf_tail_call_direct(struct bpf_prog *bpf_prog,
bool *callee_regs_used, u32 stack_depth,
struct jit_context *ctx)
{
- int tcc_off = -4 - round_up(stack_depth, 8);
+ int tcc_ptr_off = BPF_TAIL_CALL_CNT_PTR_STACK_OFF(stack_depth);
u8 *prog = *pprog, *start = *pprog;
int offset;

/*
- * if (tail_call_cnt++ >= MAX_TAIL_CALL_CNT)
+ * if ((*tcc_ptr)++ >= MAX_TAIL_CALL_CNT)
* goto out;
*/
- 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 */
+ EMIT3_off32(0x48, 0x8B, 0x85, tcc_ptr_off); /* mov rax, qword ptr [rbp - tcc_ptr_off] */
+ EMIT4(0x48, 0x83, 0x38, MAX_TAIL_CALL_CNT); /* cmp qword ptr [rax], MAX_TAIL_CALL_CNT */

offset = ctx->tail_call_direct_label - (prog + 2 - start);
EMIT2(X86_JAE, offset); /* jae out */
- EMIT3(0x83, 0xC0, 0x01); /* add eax, 1 */
- EMIT2_off32(0x89, 0x85, tcc_off); /* mov dword ptr [rbp - tcc_off], eax */

poke->tailcall_bypass = ip + (prog - start);
poke->adj_off = X86_TAIL_CALL_OFFSET;
@@ -715,6 +752,9 @@ static void emit_bpf_tail_call_direct(struct bpf_prog *bpf_prog,
emit_jump(&prog, (u8 *)poke->tailcall_target + X86_PATCH_SIZE,
poke->tailcall_bypass);

+ /* Inc tail_call_cnt if the slot is populated. */
+ EMIT4(0x48, 0x83, 0x00, 0x01); /* add qword ptr [rax], 1 */
+
if (bpf_prog->aux->exception_boundary) {
pop_callee_regs(&prog, all_callee_regs_used);
pop_r12(&prog);
@@ -724,6 +764,11 @@ static void emit_bpf_tail_call_direct(struct bpf_prog *bpf_prog,
pop_r12(&prog);
}

+ /* Pop tail_call_cnt_ptr. */
+ EMIT1(0x58); /* pop rax */
+ /* Pop tail_call_cnt, if it's main prog.
+ * Pop tail_call_cnt_ptr, if it's subprog.
+ */
EMIT1(0x58); /* pop rax */
if (stack_depth)
EMIT3_off32(0x48, 0x81, 0xC4, round_up(stack_depth, 8));
@@ -1311,9 +1356,11 @@ static void emit_shiftx(u8 **pprog, u32 dst_reg, u8 src_reg, bool is64, u8 op)

#define INSN_SZ_DIFF (((addrs[i] - addrs[i - 1]) - (prog - temp)))

-/* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
-#define RESTORE_TAIL_CALL_CNT(stack) \
- EMIT3_off32(0x48, 0x8B, 0x85, -round_up(stack, 8) - 8)
+#define __LOAD_TCC_PTR(off) \
+ EMIT3_off32(0x48, 0x8B, 0x85, off)
+/* mov rax, qword ptr [rbp - rounded_stack_depth - 16] */
+#define LOAD_TAIL_CALL_CNT_PTR(stack) \
+ __LOAD_TCC_PTR(BPF_TAIL_CALL_CNT_PTR_STACK_OFF(stack))

static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image,
int oldproglen, struct jit_context *ctx, bool jmp_padding)
@@ -2031,7 +2078,7 @@ st: if (is_imm8(insn->off))

func = (u8 *) __bpf_call_base + imm32;
if (tail_call_reachable) {
- RESTORE_TAIL_CALL_CNT(bpf_prog->aux->stack_depth);
+ LOAD_TAIL_CALL_CNT_PTR(bpf_prog->aux->stack_depth);
ip += 7;
}
if (!imm32)
@@ -2706,6 +2753,10 @@ static int invoke_bpf_mod_ret(const struct btf_func_model *m, u8 **pprog,
return 0;
}

+/* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
+#define LOAD_TRAMP_TAIL_CALL_CNT_PTR(stack) \
+ __LOAD_TCC_PTR(-round_up(stack, 8) - 8)
+
/* Example:
* __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev);
* its 'struct btf_func_model' will be nr_args=2
@@ -2826,7 +2877,7 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
* [ ... ]
* [ stack_arg2 ]
* RBP - arg_stack_off [ stack_arg1 ]
- * RSP [ tail_call_cnt ] BPF_TRAMP_F_TAIL_CALL_CTX
+ * RSP [ tail_call_cnt_ptr ] BPF_TRAMP_F_TAIL_CALL_CTX
*/

/* room for return value of orig_call or fentry prog */
@@ -2955,10 +3006,10 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
save_args(m, &prog, arg_stack_off, true);

if (flags & BPF_TRAMP_F_TAIL_CALL_CTX) {
- /* Before calling the original function, restore the
- * tail_call_cnt from stack to rax.
+ /* Before calling the original function, load the
+ * tail_call_cnt_ptr from stack to rax.
*/
- RESTORE_TAIL_CALL_CNT(stack_size);
+ LOAD_TRAMP_TAIL_CALL_CNT_PTR(stack_size);
}

if (flags & BPF_TRAMP_F_ORIG_STACK) {
@@ -3017,10 +3068,10 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
goto cleanup;
}
} else if (flags & BPF_TRAMP_F_TAIL_CALL_CTX) {
- /* Before running the original function, restore the
- * tail_call_cnt from stack to rax.
+ /* Before running the original function, load the
+ * tail_call_cnt_ptr from stack to rax.
*/
- RESTORE_TAIL_CALL_CNT(stack_size);
+ LOAD_TRAMP_TAIL_CALL_CNT_PTR(stack_size);
}

/* restore return value of orig_call or fentry prog back into RAX */
--
2.44.0