Re: [PATCH bpf-next] bpf: limit bpf_core_types_are_compat() recursion

[Yonghong Song]

On 12/17/21 11:31 AM, Matteo Croce wrote:
>   On Wed, Dec 15, 2021 at 7:21 PM Matteo Croce
> <mcroce@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Wed, Dec 15, 2021 at 6:29 PM Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> wrote:
> > > On Wed, Dec 15, 2021 at 6:54 AM Matteo Croce <mcroce@xxxxxxxxxxxxxxxxxxx> wrote:
> > > > > Maybe do a level check here?
> > > > > Since calling it and immediately returning doesn't conserve
> > > > > the stack.
> > > > > If it gets called it can finish fine, but
> > > > > calling it again would be too much.
> > > > > In other words checking the level here gives us
> > > > > room for one more frame.
> > > >
> > > > I thought that the compiler was smart enough to return before
> > > > allocating most of the frame.
> > > > I tried and this is true only with gcc, not with clang.
> > >
> > > Interesting. That's a surprise.
> > > Could you share the asm that gcc generates?
> >
> > Sure,
> >
> > This is the gcc x86_64 asm on a stripped down
> > bpf_core_types_are_compat() with a 1k struct on the stack:
> >
> > bpf_core_types_are_compat:
> > test esi, esi
> > jle .L69
> > push r15
> > push r14
> > push r13
> > push r12
> > push rbp
> > mov rbp, rdi
> > push rbx
> > mov ebx, esi
> > sub rsp, 9112
> > [...]
> > .L69:
> > or eax, -1
> > ret
> >
> > This latest clang:
> >
> > bpf_core_types_are_compat: # @bpf_core_types_are_compat
> > push rbp
> > push r15
> > push r14
> > push rbx
> > sub rsp, 1000
> > mov r14d, -1
> > test esi, esi
> > jle .LBB0_7
> > [...]
> > .LBB0_7:
> > mov eax, r14d
> > add rsp, 1000
> > pop rbx
> > pop r14
> > pop r15
> > pop rbp
> > ret
> >
> > > > > > +                       err = __bpf_core_types_are_compat(local_btf, local_id,
> > > > > > +                                                         targ_btf, targ_id,
> > > > > > +                                                         level - 1);
> > > > > > +                       if (err <= 0)
> > > > > > +                               return err;
> > > > > > +               }
> > > > > > +
> > > > > > +               /* tail recurse for return type check */
> > > > > > +               btf_type_skip_modifiers(local_btf, local_type->type, &local_id);
> > > > > > +               btf_type_skip_modifiers(targ_btf, targ_type->type, &targ_id);
> > > > > > +               goto recur;
> > > > > > +       }
> > > > > > +       default:
> > > > > > +               pr_warn("unexpected kind %s relocated, local [%d], target [%d]\n",
> > > > > > +                       btf_type_str(local_type), local_id, targ_id);
> > > > >
> > > > > That should be bpf_log() instead.
> > > >
> > > > To do that I need a struct bpf_verifier_log, which is not present
> > > > there, neither in bpf_core_spec_match() or bpf_core_apply_relo_insn().
> > >
> > > It is there. See:
> > >          err = bpf_core_apply_relo_insn((void *)ctx->log, insn, ...
> > >
> > > > Should we drop the message at all?
> > >
> > > Passing it into bpf_core_spec_match() and further into
> > > bpf_core_types_are_compat() is probably unnecessary.
> > > All callers have an error check with a log right after.
> > > So I think we won't lose anything if we drop this log.
> >
> > Nice.
> >
> > > > > > +               return 0;
> > > > > > +       }
> > > > > > +}
> > > > >
> > > > > Please add tests that exercise this logic by enabling
> > > > > additional lskels and a new test that hits the recursion limit.
> > > > > I suspect we don't have such case in selftests.
> > > > >
> > > > > Thanks!
> > > >
> > > > Will do!
> > >
> > > Thanks!
>
> Hi,
>
> I'm writing a test which exercise that function.
> I can succesfully trigger a call to __bpf_core_types_are_compat() with
> these  calls:
>
> bpf_core_type_id_kernel(struct sk_buff);
> bpf_core_type_exists(struct sk_buff);
> bpf_core_type_size(struct sk_buff);
>
> but the kind will obviously be BTF_KIND_STRUCT.
> I'm trying to do the same with kind BTF_KIND_FUNC_PROTO instead with:
>
> void func_proto(int, unsigned int);
> bpf_core_type_id_kernel(func_proto);
>
> but I get a clang crash[1], while just checking the existence with:
>
> typedef int (*func_proto_typedef)(struct sk_buff *);
> bpf_core_type_exists(func_proto_typedef);
>
> I don't reach even bpf_core_spec_match().
>
> Which is a simple way to generate a BTF_KIND_FUNC_PROTO BTF field?
>
> [1] https://github.com/llvm/llvm-project/issues/52779

Thanks for Matteo. The error message is improved in 
https://reviews.llvm.org/D116063 to make it easy to understand the 
problem. I also posted the explanation here so other people, if hitting
a similar issue, can be aware of what is going on.

The following is a simple reproducible test case:

$ cat bug.c 

extern int do_smth(int); 


int test() { 


  return __builtin_btf_type_id(*(typeof(do_smth) *)do_smth, 1); 


} 


$ clang -target bpf -O2 -g -c bug.c 

fatal error: error in backend: Empty type name for BTF_TYPE_ID_REMOTE reloc
...
Let us try to reproduce the IR to see what is really going on with command,

clang -target bpf -O2 -g bug.c -emit-llvm -S -Xclang -disable-llvm-passes
The IR,

define dso_local i32 @test() #0 !dbg !7 {
entry:
  %0 = call i64 @llvm.bpf.btf.type.id(i32 0, i64 1), !dbg !12, 
!llvm.preserve.access.index !13
  %conv = trunc i64 %0 to i32, !dbg !12
  ret i32 %conv, !dbg !15
}
...
!7 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 2, 
type: !8, scopeLine: 2, flags: DIFlagAllCallsDescribed, spFlags: 
DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !11)
!8 = !DISubroutineType(types: !9)
!9 = !{!10}
!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!11 = !{}
!12 = !DILocation(line: 3, column: 10, scope: !7)
!13 = !DISubroutineType(types: !14)
!14 = !{!10, !10}
In the above, we really try to relocate a 'subroutine' (func pointer) 
type with debuginfo id 13 which is actually "int ()(int)". There are no 
actually name for type 13 and libbpf is not able to relocate for a 
function "int ()(int)" as it could have many matches.

https://reviews.llvm.org/D116063 improved the error message as below
to make it a little bit more evident what is the problem:

$ clang -target bpf -O2 -g -c bug.c 

fatal error: error in backend: SubroutineType not supported for 
BTF_TYPE_ID_REMOTE reloc


> Regards,