On 5/1/24 12:15, Alexei Starovoitov wrote:
> On Thu, Jan 4, 2024 at 6:23 AM Leon Hwang <hffilwlqm@xxxxxxxxx> wrote:
>>
>>
>> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
>> index fe30b9ebb8de4..67fa337fc2e0c 100644
>> --- a/arch/x86/net/bpf_jit_comp.c
>> +++ b/arch/x86/net/bpf_jit_comp.c
>> @@ -259,7 +259,7 @@ 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 (22 + ENDBR_INSN_SIZE)
>>
>> static void push_r12(u8 **pprog)
>> {
>> @@ -406,14 +406,21 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
>> */
>> emit_nops(&prog, X86_PATCH_SIZE);
>> if (!ebpf_from_cbpf) {
>> - if (tail_call_reachable && !is_subprog)
>> + if (tail_call_reachable && !is_subprog) {
>> /* When it's the entry of the whole tailcall context,
>> * zeroing rax means initialising tail_call_cnt.
>> */
>> - EMIT2(0x31, 0xC0); /* xor eax, eax */
>> - else
>> - /* Keep the same instruction layout. */
>> - EMIT2(0x66, 0x90); /* nop2 */
>> + EMIT2(0x31, 0xC0); /* xor eax, eax */
>> + EMIT1(0x50); /* push rax */
>> + /* Make rax as ptr that points to tail_call_cnt. */
>> + EMIT3(0x48, 0x89, 0xE0); /* mov rax, rsp */
>> + EMIT1_off32(0xE8, 2); /* call main prog */
>> + EMIT1(0x59); /* pop rcx, get rid of tail_call_cnt */
>> + EMIT1(0xC3); /* ret */
>> + } else {
>> + /* Keep the same instruction size. */
>> + emit_nops(&prog, 13);
>> + }
>
> I'm afraid the extra call breaks stack unwinding and many other things.
> The proper frame needs to be setup (push rbp; etc)
> and 'leave' + emit_return() is used.
> Plain 'ret' is not ok.
> x86_call_depth_emit_accounting() needs to be used too.
> That will make X86_TAIL_CALL_OFFSET adjustment very complicated.
> Also the fix doesn't address the stack size issue.
> We shouldn't allow all the extra frames at run-time.
>
> The tail_cnt_ptr approach is interesting but too heavy,
> since arm64, s390 and other JITs would need to repeat it with equally
> complicated calculations in TAIL_CALL_OFFSET.
>
> The fix should really be thought through for all JITs. Not just x86.
>
> I'm thinking whether we should do the following instead:
>
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index 0bdbbbeab155..0b45571559be 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -910,7 +910,7 @@ static void *prog_fd_array_get_ptr(struct bpf_map *map,
> if (IS_ERR(prog))
> return prog;
>
> - if (!bpf_prog_map_compatible(map, prog)) {
> + if (!bpf_prog_map_compatible(map, prog) || prog->aux->func_cnt) {
> bpf_prog_put(prog);
> return ERR_PTR(-EINVAL);
> }
>
> This will stop stack growth, but it will break a few existing tests.
> I feel it's a price worth paying.
I don't think this can avoid this issue completely.
For example:
#include "vmlinux.h"
#include "bpf_helpers.h"
struct {
__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
__uint(max_entries, 1);
__uint(key_size, sizeof(__u32));
__uint(value_size, sizeof(__u32));
} prog_array SEC(".maps");
static __noinline int
subprog(struct __sk_buff *skb)
{
volatile int retval = 0;
bpf_tail_call(skb, &prog_array, 0);
return retval;
}
SEC("tc")
int entry(struct __sk_buff *skb)
{
const int N = 10000;
for (int i = 0; i < N; i++)
subprog(skb);
return 0;
}
char _license[] SEC("license") = "GPL";
Then, objdump its asm:
Disassembly of section .text:
0000000000000000 <subprog>:
; {
0: b7 02 00 00 00 00 00 00 r2 = 0x0
; volatile int retval = 0;
1: 63 2a fc ff 00 00 00 00 *(u32 *)(r10 - 0x4) = r2
; bpf_tail_call(skb, &prog_array, 0);
2: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0x0 ll
4: b7 03 00 00 00 00 00 00 r3 = 0x0
5: 85 00 00 00 0c 00 00 00 call 0xc
; return retval;
6: 61 a1 fc ff 00 00 00 00 r1 = *(u32 *)(r10 - 0x4)
7: 95 00 00 00 00 00 00 00 exit
Disassembly of section tc:
0000000000000000 <entry>:
; {
0: bf 16 00 00 00 00 00 00 r6 = r1
1: b7 07 00 00 10 27 00 00 r7 = 0x2710
0000000000000010 <LBB0_1>:
; subprog(skb);
2: bf 61 00 00 00 00 00 00 r1 = r6
3: 85 10 00 00 ff ff ff ff call -0x1
; for (int i = 0; i < N; i++)
4: 07 07 00 00 ff ff ff ff r7 += -0x1
5: bf 71 00 00 00 00 00 00 r1 = r7
6: 67 01 00 00 20 00 00 00 r1 <<= 0x20
7: 77 01 00 00 20 00 00 00 r1 >>= 0x20
8: 15 01 01 00 00 00 00 00 if r1 == 0x0 goto +0x1 <LBB0_2>
9: 05 00 f8 ff 00 00 00 00 goto -0x8 <LBB0_1>
0000000000000050 <LBB0_2>:
; return 0;
10: b7 00 00 00 00 00 00 00 r0 = 0x0
11: 95 00 00 00 00 00 00 00 exit
As a result, the bpf prog in prog_array can be tailcalled for N times,
even though there's no subprog in the bpf prog in prog_array.
Thanks,
Leon
>
> John, Daniel,
>
> do you see anything breaking on cilium side if we disallow
> progs with subprogs to be inserted in prog_array ?
>
> Other alternatives?
