Re: [PATCH bpf-next v2 1/6] kallsyms: Add KALLSYMS_QUERY ioctl to /proc/kallsyms

[David Laight]

On Thu, 29 Oct 2026 17:28:01 -0700
Samuel Wu <wusamuel@xxxxxxxxxx> wrote:

> On Thu, Oct 29, 2026 at 3:16 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Thu, Oct 29, 2026 at 3:06 PM Samuel Wu <wusamuel@xxxxxxxxxx> wrote:  
> > >
> > > BY_NAME matches names exactly, like kallsyms_lookup_name(), so they
> > > would be different symbols at different addresses. Please correct me
> > > if I'm missing something here!
> > >  
> >
> > foo and foo.constprop0 are different functions with different
> > arguments, so I think we should distinguish them. The only special
> > case is .llvm.<hash> because that suffix is added by compiler for LTO
> > and I don't think the function itself is any different from the
> > original  
> 
> [...]
> 
> > I do think it would be good to spend a bit of thought on how to
> > support at least one-by-one pagination from the get go. The same
> > mechanism can then be used by batch version. I.e., think what can we
> > use to provide data back to user which they can use to continue search
> > after the symbol we just returned (at least for fixed set of symbols
> > in vmlinux and modules; we also have dynamic symbols added for [bpf]
> > "module", etc, we should support those as much as possible with the
> > least possible deviation, IMO)  
> 
> Let me sit on this a bit and figure out how to distill all of this
> into a clean interface. I'll keep the .llvm.<hash>, pagination, and
> batching in mind for the design.
> 
> [...]
> 
> > user code can be easily written by always setting buffer size, even if
> > kallsyms_query is reused. This is the pattern with PROCMAP_QUERY,
> > let's keep it consistent.  
> 
> Ack, will have size in the struct for v3.

It just adds code.
If you use the size from the ioctl (which will match the structure
the application passes) then you don't need to do the extra get_user()
just to get the length.
It isn't really hard to have:
	case KALLSYMS_QUERY_v1:
	case KALLSYMS_QUERY:
		return do_kallsys_query(m->private, cmd, (void __user *)arg)
It also saves all the problems of the two values mismatching.

That is why there is a length (and the r/w flags) embedded in the ioctl
cmd value.

David

> 
> > also, returning actual size even if buffer is too short is a good way
> > for user space to "probe" and increase buffer if necessary. unless it
> > adds tons of complexity, this is a good and flexible UAPI  
> 
> Ack.