On Thu, Dec 10, 2020 at 9:02 AM Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > On Thu, Dec 10, 2020 at 8:43 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: > > > > On Wed, Dec 09, 2020 at 12:53:44PM -0800, Andrii Nakryiko wrote: > > > Hi, > > > > > > I'm working on supporting per-CPU symbols in BPF/libbpf, and the > > > prerequisite for that is BTF data for .data..percpu data section and > > > variables inside that. > > > > > > Turns out, pahole doesn't currently emit any BTF information for such > > > variables in kernel modules. And the reason why is quite confusing and > > > I can't figure it out myself, so was hoping someone else might be able > > > to help. > > > > > > To repro, you can take latest bpf-next tree and add this to > > > bpf_testmod/bpf_testmod.c inside selftests/bpf: > > > > > > $ git diff bpf_testmod/bpf_testmod.c > > > diff --git > > > a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c > > > b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c > > > index 2df19d73ca49..b2086b798019 100644 > > > --- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c > > > +++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c > > > @@ -3,6 +3,7 @@ > > > #include <linux/error-injection.h> > > > #include <linux/init.h> > > > #include <linux/module.h> > > > +#include <linux/percpu-defs.h> > > > #include <linux/sysfs.h> > > > #include <linux/tracepoint.h> > > > #include "bpf_testmod.h" > > > @@ -10,6 +11,10 @@ > > > #define CREATE_TRACE_POINTS > > > #include "bpf_testmod-events.h" > > > > > > +DEFINE_PER_CPU(int, bpf_testmod_ksym_dummy1) = -1; > > > +DEFINE_PER_CPU(int, bpf_testmod_ksym_percpu) = 123; > > > +DEFINE_PER_CPU(int, bpf_testmod_ksym_dummy2) = -1; > > > + > > > noinline ssize_t > > > bpf_testmod_test_read(struct file *file, struct kobject *kobj, > > > struct bin_attribute *bin_attr, > > > > > > 1. So the very first issue (that I'm going to ignore for now) is that > > > if I just added bpf_testmod_ksym_percpu, it would get addr == 0 and > > > would be ignored by the current pahole logic. So we need to fix that > > > for modules. Adding dummy1 and dummy2 takes care of this for now, > > > bpf_testmod_ksym_percpu has offset 4. > > > > I removed that addr zero check in the modules changes but when > > collecting functions, but it's still there in collect_percpu_var > > Hao had some reason to skip per-cpu variables with offset 0, maybe he > can comment on that before we change it. > When I initially write that check, I see there are multiple symbols of the same name that associate with a single variable, but there is only one that has a non-zero address. Besides, there are symbols that don't associate to any variable and they have zero address. For example, those defined as __ADDRESSABLE(sym) and __UNIQUE_ID(prefix). They are quite a lot, I remember. So I filtered out the zero address for the purpose of accelerating encoding. I noticed that on x86_64, the first page of the percpu section is reserved, so I deem those symbols that are of normal interest should have positive addresses. > > > > > > > > > 2. Second issue is more interesting. Somehow, when pahole iterates > > > over DWARF variables, the address of bpf_testmod_ksym_percpu is > > > reported as 0x10e74, not 4. Which totally confuses pahole because > > > according to ELF symbols, bpf_testmod_ksym_percpu symbol has value 4. > > > I tracked this down to dwarf_getlocation() returning 10e74 as number > > > field in expr. > > > > in which place do you see that address? when I put displayed > > address from collect_percpu_var it shows 4 > > yes, ELF symbol's value is 4, but when iterating DWARF variables > (0x10e70 + 4) is returned. It does look like a special handling of > modules. I missed that libdw does some special things for specifically > modules. Further debugging yesterday showed that 0x10e70 roughly > corresponds to the offset of .data..per_cpu if you count all the > allocatable data sections that come before it. So I think you are > right. We should probably centralize the logic of kernel module > detection so that we can handle these module vs non-module differences > properly. > > > > > not sure this is related but looks like similar issue I had to > > solve for modules functions, as described in the changelog: > > (not merged yet) > > > > btf_encoder: Detect kernel module ftrace addresses > > > > ... > > There's one tricky point with kernel modules wrt Elf object, > > which we get from dwfl_module_getelf function. This function > > performs all possible relocations, including __mcount_loc > > section. > > > > So addrs array contains relocated values, which we need take > > into account when we compare them to functions values which > > are relative to their sections. > > ... > > > > The 0x10e74 value could be relocated 4.. but it's me guessing, > > because not sure where you see that address exactly > > > It comes up in cu__encode_btf(), var->ip.addr is not 4, as we expect it to be. > > > > > jirka > >
