On 6/10/23 02:05, Stanislav Fomichev wrote:
> On 10/05, Leon Hwang wrote:
>> From commit ebf7d1f508a73871 ("bpf, x64: rework pro/epilogue and tailcall
>> handling in JIT"), the tailcall on x64 works better than before.
>>
>> From commit e411901c0b775a3a ("bpf: allow for tailcalls in BPF subprograms
>> for x64 JIT"), tailcall is able to run in BPF subprograms on x64.
>>
>> How about:
>>
>> 1. More than 1 subprograms are called in a bpf program.
>> 2. The tailcalls in the subprograms call the bpf program.
>>
>> Because of missing tail_call_cnt back-propagation, a tailcall hierarchy
>> comes up. And MAX_TAIL_CALL_CNT limit does not work for this case.
>>
>> As we know, in tail call context, the tail_call_cnt propagates by stack
>> and rax register between BPF subprograms. So, propagating tail_call_cnt
>> pointer by stack and rax register makes tail_call_cnt as like a global
>> variable, in order to make MAX_TAIL_CALL_CNT limit works for tailcall
>> hierarchy cases.
>>
>> Before jumping to other bpf prog, load tail_call_cnt from the pointer
>> and then compare with MAX_TAIL_CALL_CNT. Finally, increment
>> tail_call_cnt by the pointer.
>>
>> But, where does tail_call_cnt store?
>>
>> It stores on the stack of uppest-hierarchy-layer bpf prog, like
>>
>> | STACK |
>> +---------+ RBP
>> | |
>> | |
>> | |
>> | tcc_ptr |
>> | tcc |
>> | rbx |
>> +---------+ RSP
>>
>> Why not back-propagate tail_call_cnt?
>>
>> It's because it's vulnerable to back-propagate it. It's unable to work
>> well with the following case.
>>
>> int prog1();
>> int prog2();
>>
>> prog1 is tail caller, and prog2 is tail callee. If we do back-propagate
>> tail_call_cnt at the epilogue of prog2, can prog2 run standalone at the
>> same time? The answer is NO. Otherwise, there will be a register to be
>> polluted, which will make kernel crash.
>>
>> Can tail_call_cnt store at other place instead of the stack of bpf prog?
>>
>> I'm not able to infer a better place to store tail_call_cnt. It's not a
>> working inference to store it at ctx or on the stack of bpf prog's
>> caller.
>>
>> 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@xxxxxxxxx>
>> ---
>> arch/x86/net/bpf_jit_comp.c | 120 +++++++++++++++++++++++-------------
>> 1 file changed, 76 insertions(+), 44 deletions(-)
>>
>> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
>> index 8c10d9abc2394..8ad6368353c2b 100644
>> --- a/arch/x86/net/bpf_jit_comp.c
>> +++ b/arch/x86/net/bpf_jit_comp.c
>> @@ -256,7 +256,7 @@ 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 (24 + ENDBR_INSN_SIZE)
>>
>> static void push_r12(u8 **pprog)
>> {
>> @@ -304,6 +304,25 @@ static void pop_callee_regs(u8 **pprog, bool *callee_regs_used)
>> *pprog = prog;
>> }
>>
>
> [..]
>
>> +static void emit_nops(u8 **pprog, int len)
>> +{
>> + u8 *prog = *pprog;
>> + int i, noplen;
>> +
>> + while (len > 0) {
>> + noplen = len;
>> +
>> + if (noplen > ASM_NOP_MAX)
>> + noplen = ASM_NOP_MAX;
>> +
>> + for (i = 0; i < noplen; i++)
>> + EMIT1(x86_nops[noplen][i]);
>> + len -= noplen;
>> + }
>> +
>> + *pprog = prog;
>> +}
>
> From high level - makes sense to me.
> I'll leave a thorough review to the people who understand more :-)
> I see Maciej commenting on your original "Fix tailcall infinite loop"
> series.
Welcome for your review.
>
> One suggestion I have is: the changes to 'memcpy(prog, x86_nops[5],
> X86_PATCH_SIZE);' and this emit_nops move here don't seem like
> they actually belong to this patch. Maybe do them separately?
Moving emit_nops here is for them:
+ /* Keep the same instruction layout. */
+ emit_nops(&prog, 3);
+ emit_nops(&prog, 6);
+ emit_nops(&prog, 6);
and do the changes to 'memcpy(prog, x86_nops[5], X86_PATCH_SIZE);' BTW.
Thanks,
Leon
