Re: [PATCH bpf-next v9 4/9] bpf: Add syscall common attributes support for prog_load

[Leon Hwang <leon.hwang@linux.dev>]

On 5/2/26 03:48, Andrii Nakryiko wrote:
> On Mon, Feb 2, 2026 at 6:42 AM Leon Hwang <leon.hwang@linux.dev> wrote:
>>
>> BPF_PROG_LOAD can now provide log parameters through both union bpf_attr
>> and struct bpf_common_attr. Define clear conflict and precedence rules:
>>
>> - if both are provided and log_buf/log_size/log_level match, use them;
>> - if only one side provides a log buffer, use that one;
>> - if both provide log buffers but differ, return -EINVAL.
>>
>> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
>> ---
>>  include/linux/bpf_verifier.h |  3 ++-
>>  kernel/bpf/log.c             | 24 ++++++++++++++++++++++--
>>  kernel/bpf/syscall.c         |  3 ++-
>>  3 files changed, 26 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
>> index c805b85b6f7a..0d106fddbbc5 100644
>> --- a/include/linux/bpf_verifier.h
>> +++ b/include/linux/bpf_verifier.h
>> @@ -638,7 +638,8 @@ struct bpf_log_attr {
>>  };
>>
>>  int bpf_prog_load_log_attr_init(struct bpf_log_attr *attr_log, union bpf_attr *attr,
>> -                               bpfptr_t uattr, u32 size);
>> +                               bpfptr_t uattr, u32 size, struct bpf_common_attr *attr_common,
>> +                               bpfptr_t uattr_common, u32 size_common);
>>  int bpf_log_attr_finalize(struct bpf_log_attr *attr, struct bpf_verifier_log *log);
>>
>>  #define BPF_MAX_SUBPROGS 256
>> diff --git a/kernel/bpf/log.c b/kernel/bpf/log.c
>> index ff579fcba36f..345005ba98dd 100644
>> --- a/kernel/bpf/log.c
>> +++ b/kernel/bpf/log.c
>> @@ -873,10 +873,30 @@ static void bpf_log_attr_init(struct bpf_log_attr *attr_log, int offsetof_true_s
>>         attr_log->uattr = uattr;
>>  }
>>
>> +static bool bpf_log_attrs_diff(struct bpf_common_attr *common, u64 log_buf, u32 log_size,
>> +                              u32 log_level)
>> +{
>> +       return log_buf && common->log_buf && (log_buf != common->log_buf ||
>> +                                             log_size != common->log_size ||
>> +                                             log_level != common->log_level);
> 
> let's validate (unless we do this somewhere else) that if log_buf is
> set, then log_size and log_level (? not sure, maybe zero is fine) are
> set, or all three are not set. Same for common->log* fields...
> 

Ack.

Will validate 'log_buf && log_size && log_level' first.

> 
>> +}
>> +
>>  int bpf_prog_load_log_attr_init(struct bpf_log_attr *attr_log, union bpf_attr *attr,
>> -                               bpfptr_t uattr, u32 size)
>> +                               bpfptr_t uattr, u32 size, struct bpf_common_attr *attr_common,
>> +                               bpfptr_t uattr_common, u32 size_common)
>>  {
>> -       bpf_log_attr_init(attr_log, offsetof(union bpf_attr, log_true_size), uattr, size);
>> +       if (bpf_log_attrs_diff(attr_common, attr->log_buf, attr->log_size, attr->log_level))
>> +               return -EINVAL;
>> +
>> +       if (!attr->log_buf && attr_common->log_buf) {
>> +               attr->log_buf = attr_common->log_buf;
>> +               attr->log_size = attr_common->log_size;
>> +               attr->log_level = attr_common->log_level;
> 
> why are we setting this? Do we still have code that can access
> attr->log_buf even though we pass attr_log everywhere? If yes, should
> we still have that "split brain" code?
> 

'attr->log_buf' is accessed only in bpf_check().

> If we don't have this assignment, then I think we don't need to have
> bpf_prog_load-specific and btf_load-specific log_attr_init() helpers.
> They can be unified into generic log_attr_init, where for
> bpf_prog_load you'll pass offsetof(log_true_size) +
> attr->log_{buf,size,level}, and for btf_load you'll pass different
> offset of and btf-specific attr->btf_log*
> 
> This helper will just be making decision whether to use common_attr's
> log fields or passed directly command-specific ones.
> 
> Or what am I missing?
> 

If the log attributes differ, where should the effective
log_* values be stored?

Should they live in struct bpf_common_attr, or should we extend
struct bpf_log_attr to carry them?

Note that in v8, Alexei suggested struct bpf_log_attr only needs
  u32 offsetof_true_size;
  bpfptr_t uattr;

so I’d like to clarify the intended direction here. Once that’s clear, a
single generic log_attr_init() should be sufficient to handle this.

Thanks,
Leon

> 
>> +               bpf_log_attr_init(attr_log, offsetof(struct bpf_common_attr, log_true_size),
>> +                                 uattr_common, size_common);
>> +       } else {
>> +               bpf_log_attr_init(attr_log, offsetof(union bpf_attr, log_true_size), uattr, size);
>> +       }
>>         return 0;
>>  }
>>
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index e81199361241..7125ea445c6c 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -6232,7 +6232,8 @@ static int __sys_bpf(enum bpf_cmd cmd, bpfptr_t uattr, unsigned int size,
>>                 err = map_freeze(&attr);
>>                 break;
>>         case BPF_PROG_LOAD:
>> -               err = bpf_prog_load_log_attr_init(&attr_log, &attr, uattr, size);
>> +               err = bpf_prog_load_log_attr_init(&attr_log, &attr, uattr, size, &attr_common,
>> +                                                 uattr_common, size_common);
>>                 err = err ?: bpf_prog_load(&attr, uattr, &attr_log);
>>                 break;
>>         case BPF_OBJ_PIN:
>> --
>> 2.52.0
>>