On Wed, Mar 10, 2021 at 8:59 AM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 3/10/21 3:48 AM, Florent Revest wrote:
> > On Wed, Mar 10, 2021 at 6:16 AM Yonghong Song <yhs@xxxxxx> wrote:
> >> On 3/9/21 7:43 PM, Yonghong Song wrote:
> >>> On 3/9/21 5:54 PM, Florent Revest wrote:
> >>>> I noticed that initializing an array of pointers using this syntax:
> >>>> __u64 array[] = { (__u64)&var1, (__u64)&var2 };
> >>>> (which is a fairly common operation with macros such as BPF_SEQ_PRINTF)
> >>>> always results in array[0] and array[1] being NULL.
> >>>>
> >>>> Interestingly, if the array is only initialized with one pointer, ex:
> >>>> __u64 array[] = { (__u64)&var1 };
> >>>> Then array[0] will not be NULL.
> >>>>
> >>>> Or if the array is initialized field by field, ex:
> >>>> __u64 array[2];
> >>>> array[0] = (__u64)&var1;
> >>>> array[1] = (__u64)&var2;
> >>>> Then array[0] and array[1] will not be NULL either.
> >>>>
> >>>> I'm assuming that this should have something to do with relocations
> >>>> and might be a bug in clang or in libbpf but because I don't know much
> >>>> about these, I thought that reporting could be a good first step. :)
> >>>
> >>> Thanks for reporting. What you guess is correct, this is due to
> >>> relocations :-(
> >>>
> >>> The compiler notoriously tend to put complex initial values into
> >>> rodata section. For example, for
> >>> __u64 array[] = { (__u64)&var1, (__u64)&var2 };
> >>> the compiler will put
> >>> { (__u64)&var1, (__u64)&var2 }
> >>> into rodata section.
> >>>
> >>> But &var1 and &var2 themselves need relocation since they are
> >>> address of static variables which will sit inside .data section.
> >>>
> >>> So in the elf file, you will see the following relocations:
> >>>
> >>> RELOCATION RECORDS FOR [.rodata]:
> >>> OFFSET TYPE VALUE
> >>> 0000000000000018 R_BPF_64_64 .data
> >>> 0000000000000020 R_BPF_64_64 .data
> >
> > Right :) Thank you for the explanations Yonghong!
> >
> >>> Currently, libbpf does not handle relocation inside .rodata
> >>> section, so they content remains 0.
> >
> > Just for my own edification, why is .rodata relocation not yet handled
> > in libbpf ? Is it because of a read-only mapping that makes it more
> > difficult ?
>
> We don't have this use case before. In general, people do not put
> string pointers in init code in the declaration. I think
> bpf_seq_printf() is special about this and hence triggering
> the issue.
>
> To support relocation of rodata section, kernel needs to be
> involved and this is actually more complicated as
Exactly. It would be trivial for libbpf to support it, but it needs to
resolve to the actual in-kernel address of a map (plus offset), which
libbpf has no way of knowing.
> the relocation is against .data section. Two issues the kernel
> needs to deal with:
> - .data section will be another map in kernel, so i.e.,
> relocation of .rodata map value against another map.
> - .data section may be modified, some protection might
> be needed to prevent this. We may ignore this requirement
> since user space may have similar issue.
>
> This is a corner case, if we can workaround in the libbpf, in
> this particular case, bpf_tracing.h. I think it will be
> good enough, not adding further complexity in kernel for
> such a corner case.
Is there some way to trick compiler into thinking that those values
are not constant? Some volatile and pointers game? Or any other magic?
>
> >
> >>> That is why you see the issue with pointer as NULL.
> >>>
> >>> With array size of 1, compiler does not bother to put it into
> >>> rodata section.
> >>>
> >>> I *guess* that it works in the macro due to some kind of heuristics,
> >>> e.g., nested blocks, etc, and llvm did not promote the array init value
> >>> to rodata. I will double check whether llvm can complete prevent
> >>> such transformation.
> >>>
> >>> Maybe in the future libbpf is able to handle relocations for
> >>> rodata section too. But for the time being, please just consider to use
> >>> either macro, or the explicit array assignment.
> >>
> >> Digging into the compiler, the compiler tries to make *const* initial
> >> value into rodata section if the initial value size > 64, so in
> >> this case, macro does not work either. I think this is how you
> >> discovered the issue.
> >
> > Indeed, I was using a macro similar to BPF_SEQ_PRINTF and this is how
> > I found the bug.
> >
> >> The llvm does not provide target hooks to
> >> influence this transformation.
> >
> > Oh, that is unfortunate :) Thanks for looking into it! I feel that the
> > real fix would be in libbpf anyway and the rest is just workarounds.
>
> The real fix will need libbpf and kernel.
>
> >
> >> So, there are two workarounds,
> >> (1). __u64 param_working[2];
> >> param_working[0] = (__u64)str1;
> >> param_working[1] = (__u64)str2;
> >> (2). BPF_SEQ_PRINTF(seq, "%s ", str1);
> >> BPF_SEQ_PRINTF(seq, "%s", str2);
> >
> > (2) is a bit impractical for my actual usecase. I am implementing a
> > bpf_snprintf helper (patch series Coming Soon TM) and I wanted to keep
> > the selftest short with a few BPF_SNPRINTF() calls that exercise most
> > format specifiers.
> >
> >> In practice, if you have at least one non-const format argument,
> >> you should be fine. But if all format arguments are constant, then
> >> none of them should be strings.
> >
> > Just for context, this does not only happen for strings but also for
> > all sorts of pointers, for example, when I try to do address lookup of
> > global __ksym variables, which is important for my selftest.
>
> Currently, in bpf_seq_printf(), we do memory copy for string
> and certain ipv4/ipv6 addresses. ipv4 is not an issue as the compiler
> less likely put it into rodata. for ipv6,
> if it is a constant, we can just directly put it into the format
> string. For many other sort of pointers, we just print pointer
> values, I don't see a value to print pointer value for something like
> static const param[] = { &str1, &str2 };
> bpf_seq_printf(seq, "%px\n", param[0]);
>
> The global __ksym variable cannot be pointing to rodata at compile time,
> so it should be fine.
>
> >
> >> Maybe we could change marco
> >> unsigned long long ___param[] = { args };
> >> to declare an array explicitly and then have a loop to
> >> assign each array element?
> >
> > I think this would be a good workaround for now, indeed. :) I'll look
> > into it today and send it as part of my bpf_snprintf series.
>
> If we can make it work, that will be great! thanks for working on this.
>
> >
> > Thanks!
> >
