Re: [PATCH bpf v2 6/6] bpf: fix JITed dump for multi-function programs via syscall

[Sandipan Das <sandipan@linux.vnet.ibm.com>]

Hi Daniel,

On 05/18/2018 09:21 PM, Daniel Borkmann wrote:
> On 05/18/2018 02:50 PM, Sandipan Das wrote:
>> Currently, for multi-function programs, we cannot get the JITed
>> instructions using the bpf system call's BPF_OBJ_GET_INFO_BY_FD
>> command. Because of this, userspace tools such as bpftool fail
>> to identify a multi-function program as being JITed or not.
>>
>> With the JIT enabled and the test program running, this can be
>> verified as follows:
>>
>>   # cat /proc/sys/net/core/bpf_jit_enable
>>   1
>>
>> Before applying this patch:
>>
>>   # bpftool prog list
>>   1: kprobe  name foo  tag b811aab41a39ad3d  gpl
>>           loaded_at 2018-05-16T11:43:38+0530  uid 0
>>           xlated 216B  not jited  memlock 65536B
>>   ...
>>
>>   # bpftool prog dump jited id 1
>>   no instructions returned
>>
>> After applying this patch:
>>
>>   # bpftool prog list
>>   1: kprobe  name foo  tag b811aab41a39ad3d  gpl
>>           loaded_at 2018-05-16T12:13:01+0530  uid 0
>>           xlated 216B  jited 308B  memlock 65536B
>>   ...
> 
> That's really nice! One comment inline below:
> 
>>   # bpftool prog dump jited id 1
>>      0:   nop
>>      4:   nop
>>      8:   mflr    r0
>>      c:   std     r0,16(r1)
>>     10:   stdu    r1,-112(r1)
>>     14:   std     r31,104(r1)
>>     18:   addi    r31,r1,48
>>     1c:   li      r3,10
>>   ...
>>
>> Signed-off-by: Sandipan Das <sandipan@linux.vnet.ibm.com>
>> ---
>>  kernel/bpf/syscall.c | 38 ++++++++++++++++++++++++++++++++------
>>  1 file changed, 32 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index 54a72fafe57c..2430d159078c 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -1896,7 +1896,7 @@ static int bpf_prog_get_info_by_fd(struct bpf_prog *prog,
>>  	struct bpf_prog_info info = {};
>>  	u32 info_len = attr->info.info_len;
>>  	char __user *uinsns;
>> -	u32 ulen;
>> +	u32 ulen, i;
>>  	int err;
>>  
>>  	err = check_uarg_tail_zero(uinfo, sizeof(info), info_len);
>> @@ -1922,7 +1922,6 @@ static int bpf_prog_get_info_by_fd(struct bpf_prog *prog,
>>  	ulen = min_t(u32, info.nr_map_ids, ulen);
>>  	if (ulen) {
>>  		u32 __user *user_map_ids = u64_to_user_ptr(info.map_ids);
>> -		u32 i;
>>  
>>  		for (i = 0; i < ulen; i++)
>>  			if (put_user(prog->aux->used_maps[i]->id,
>> @@ -1970,13 +1969,41 @@ static int bpf_prog_get_info_by_fd(struct bpf_prog *prog,
>>  	 * for offload.
>>  	 */
>>  	ulen = info.jited_prog_len;
>> -	info.jited_prog_len = prog->jited_len;
>> +	if (prog->aux->func_cnt) {
>> +		info.jited_prog_len = 0;
>> +		for (i = 0; i < prog->aux->func_cnt; i++)
>> +			info.jited_prog_len += prog->aux->func[i]->jited_len;
>> +	} else {
>> +		info.jited_prog_len = prog->jited_len;
>> +	}
>> +
>>  	if (info.jited_prog_len && ulen) {
>>  		if (bpf_dump_raw_ok()) {
>>  			uinsns = u64_to_user_ptr(info.jited_prog_insns);
>>  			ulen = min_t(u32, info.jited_prog_len, ulen);
>> -			if (copy_to_user(uinsns, prog->bpf_func, ulen))
>> -				return -EFAULT;
>> +
>> +			/* for multi-function programs, copy the JITed
>> +			 * instructions for all the functions
>> +			 */
>> +			if (prog->aux->func_cnt) {
>> +				u32 len, free;
>> +				u8 *img;
>> +
>> +				free = ulen;
>> +				for (i = 0; i < prog->aux->func_cnt; i++) {
>> +					len = prog->aux->func[i]->jited_len;
>> +					img = (u8 *) prog->aux->func[i]->bpf_func;
>> +					if (len > free)
>> +						break;
>> +					if (copy_to_user(uinsns, img, len))
>> +						return -EFAULT;
>> +					uinsns += len;
>> +					free -= len;
> 
> Is there any way we can introduce a delimiter between the different
> images such that they could be more easily correlated with the call
> from the main (or other sub-)program instead of having one contiguous
> dump blob?
> 

Can we have another member in bpf_prog_info that points to a list of the lengths of the
JITed images for each subprogram? We can use this information to split up the dump.

- Sandipan 

>> +				}
>> +			} else {
>> +				if (copy_to_user(uinsns, prog->bpf_func, ulen))
>> +					return -EFAULT;
>> +			}
>>  		} else {
>>  			info.jited_prog_insns = 0;
>>  		}
>> @@ -1987,7 +2014,6 @@ static int bpf_prog_get_info_by_fd(struct bpf_prog *prog,
>>  	if (info.nr_jited_ksyms && ulen) {
>>  		u64 __user *user_jited_ksyms = u64_to_user_ptr(info.jited_ksyms);
>>  		ulong ksym_addr;
>> -		u32 i;
>>  
>>  		/* copy the address of the kernel symbol corresponding to
>>  		 * each function
>>
> 
>