Re: [PATCH v3] btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.

[Andrii Nakryiko <andrii.nakryiko-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Fri, Jun 12, 2020 at 2:54 PM Hao Luo <haoluo-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
>
> Further updates:
>
> Previously the in-kernel bpf verifier rejected the enhanced vmlinux because I set the "size" field of datasec to 0, which is obviously forbidden by the bpf kernel verifier. After I adjusted it to the last var_secinfo's offset + size, it got loaded successfully. In addition, there are a few more sanity checks in the verifier on DATASEC and VAR's meta format (e.g. type size, variable name, etc.), which I am going to port into btf_encoder to be 100% safe. With these checks, the "(anon)" vars seen by Arnaldo should be gone. I am currently running through a set of tp_btf, fentry and fexit programs on the enhanced vmlinux and they are looking good so far. I hope to upload these changes in the next iteration next week.

Do you know where those (anon) vars are coming from?

>
> Thanks,
> Hao
>
> On Thu, Jun 11, 2020 at 2:41 PM Hao Luo <haoluo-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
>>
>> I am finally able to get a tp_btf program compiled and tested against the generated vmlinux. Unfortunately, the bpf verifier seemed to have rejected the vmlinux. I got an error message "in-kernel BTF is malformed". I have to work on the bpf verifier first to make it compatible with the newly added VARs.
>>
>> Hao
>>
>> On Thu, Jun 11, 2020 at 10:59 AM Hao Luo <haoluo-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
>>>
>>> Hi Arnaldo,
>>>
>>> Sorry for the late reply, I was tied to other stuff on my other work in the last couple of days. I am going to take a closer look today and tomorrow. It seems I had difficulty reproducing in my local environment, maybe due to differences in compiler flags or kconfigs. Could you help me by enabling verbose to see which CU generated those symbols? In the patch I have added debug messages reporting the current CU and symbols that got encoded.
>>>
>>> Thanks,
>>> Hao
>>>
>>>
>>> On Tue, Jun 9, 2020 at 7:58 AM Arnaldo Carvalho de Melo <acme-DgEjT+Ai2yhQFI55V6+gNQ@public.gmane.orgg> wrote:
>>>>
>>>> Em Tue, Jun 09, 2020 at 11:29:40AM -0300, Arnaldo Carvalho de Melo escreveu:
>>>> > Em Mon, Jun 08, 2020 at 10:34:03AM -0700, Hao Luo escreveu:
>>>> > > On SMP systems, the global percpu variables are placed in a special
>>>> > > '.data..percpu' section, which is stored in a segment whose initial
>>>> > > address is set to 0, the addresses of per-CPU variables are relative
>>>> > > positive addresses [1].
>>>> > >
>>>> > > This patch extracts these variables from vmlinux and places them with
>>>> > > their type information in BTF. More specifically, when BTF is encoded,
>>>> > > we find the index of the '.data..percpu' section and then traverse
>>>> > > the symbol table to find those global objects which are in this section.
>>>> > > For each of these objects, we push a BTF_KIND_VAR into the types buffer,
>>>> > > and a BTF_VAR_SECINFO into another buffer, percpu_secinfo. When all the
>>>> > > CUs have finished processing, we push a BTF_KIND_DATASEC into the
>>>> > > btfe->types buffer, followed by the percpu_secinfo's content.
>>>> > >
>>>> > > In a v5.7-rc7 linux kernel, I was able to extract 291 such variables.
>>>> > > The build time overhead is small and the space overhead is also small.
>>>> >
>>>> > Looks good, I'm doing some testing on it now, Andrii, can you provide an
>>>> > Acked-by or Reviewed-by?
>>>>
>>>> So, I see these (anon) variables, what are these? for an 5.7 vmlinux:
>>>>
>>>> [acme@five pahole]$ bpftool btf dump file vmlinux | grep -w VAR | tail
>>>> [67381] VAR '(anon)' type_id=67175, linkage=static
>>>> [67495] VAR 'rt_cache_stat' type_id=67417, linkage=static
>>>> [67496] VAR 'rt_uncached_list' type_id=67416, linkage=static
>>>> [67857] VAR 'tcp_md5sig_pool' type_id=67788, linkage=static
>>>> [67993] VAR 'tsq_tasklet' type_id=67927, linkage=static
>>>> [69524] VAR 'xfrm_trans_tasklet' type_id=69502, linkage=static
>>>> [70055] VAR 'rt6_uncached_list' type_id=67416, linkage=static
>>>> [70609] VAR '(anon)' type_id=1713, linkage=static
>>>> [70634] VAR 'hmac_ring' type_id=1909, linkage=static
>>>> [71591] VAR 'xskmap_flush_list' type_id=85, linkage=static
>>>> [acme@five pahole]$
>>>>
>>>> [acme@five pahole]$ bpftool btf dump file vmlinux | grep -w 1713
>>>> [1713] FUNC 'memset' type_id=1712
>>>> [7235] VAR '(anon)' type_id=1713, linkage=static
>>>> [7236] VAR '(anon)' type_id=1713, linkage=static
>>>> [8832] VAR '(anon)' type_id=1713, linkage=static
>>>> [14346] VAR '(anon)' type_id=1713, linkage=static
>>>> [14347] VAR '(anon)' type_id=1713, linkage=static
>>>> [14348] VAR '(anon)' type_id=1713, linkage=static
>>>> [14349] VAR '(anon)' type_id=1713, linkage=static
>>>> [14350] VAR '(anon)' type_id=1713, linkage=static
>>>> [14351] VAR '(anon)' type_id=1713, linkage=static
>>>> [14352] VAR '(anon)' type_id=1713, linkage=static
>>>> [18903] VAR '(anon)' type_id=1713, linkage=static
>>>> [18904] VAR '(anon)' type_id=1713, linkage=static
>>>> [23180] VAR '(anon)' type_id=1713, linkage=static
>>>> [44605] VAR '(anon)' type_id=1713, linkage=static
>>>> [60638] VAR '(anon)' type_id=1713, linkage=static
>>>> [60639] VAR '(anon)' type_id=1713, linkage=static
>>>> [63869] VAR '(anon)' type_id=1713, linkage=static
>>>> [70609] VAR '(anon)' type_id=1713, linkage=static
>>>> [acme@five pahole]$
>>>>
>>>> [acme@five pahole]$ bpftool btf dump file vmlinux | grep -m1 -w 1713 -B9
>>>> [1710] FUNC_PROTO '(anon)' ret_type_id=96 vlen=3
>>>>         'p' type_id=96
>>>>         'q' type_id=97
>>>>         'size' type_id=49
>>>> [1711] FUNC 'memcpy' type_id=1710
>>>> [1712] FUNC_PROTO '(anon)' ret_type_id=96 vlen=3
>>>>         'p' type_id=96
>>>>         'c' type_id=22
>>>>         'size' type_id=49
>>>> [1713] FUNC 'memset' type_id=1712
>>>> [acme@five pahole]$
>>>>
>>>> [acme@five pahole]$ bpftool btf dump file vmlinux | grep -m10 -w 7235 -B5 -A5
>>>>         'prec' type_id=22
>>>> [7232] FUNC 'arch_show_interrupts' type_id=7231
>>>> [7233] FUNC_PROTO '(anon)' ret_type_id=0 vlen=1
>>>>         'irq' type_id=10
>>>> [7234] FUNC 'ack_bad_irq' type_id=7233
>>>> [7235] VAR '(anon)' type_id=1713, linkage=static
>>>> [7236] VAR '(anon)' type_id=1713, linkage=static
>>>> [7237] ARRAY '(anon)' type_id=233 index_type_id=1 nr_elems=4
>>>> [7238] FUNC 'irq_init_percpu_irqstack' type_id=6732
>>>> [7239] VAR 'irq_stack_backing_store' type_id=412, linkage=global-alloc
>>>> [7240] STRUCT 'estack_pages' size=8 vlen=3
>>>> --
>>>>         type_id=6739 offset=98096 size=16
>>>>         type_id=6737 offset=98112 size=16
>>>>         type_id=6736 offset=98128 size=16
>>>>         type_id=6764 offset=98144 size=16
>>>>         type_id=6763 offset=98160 size=16
>>>>         type_id=7235 offset=98176 size=0
>>>>         type_id=7330 offset=98192 size=4
>>>>         type_id=7329 offset=98200 size=8
>>>>         type_id=7328 offset=98208 size=4
>>>>         type_id=7325 offset=98216 size=8
>>>>         type_id=7327 offset=98224 size=1
>>>> [acme@five pahole]$
>>>>
>>>> [acme@five pahole]$ readelf  -wi vmlinux  | grep -m 2 DW_AT_producer
>>>>     <1c>   DW_AT_producer    : (indirect string, offset: 0x49): GNU AS 2.32
>>>>     <2e>   DW_AT_producer    : (indirect string, offset: 0x1358): GNU C89 9.3.1 20200408 (Red Hat 9.3.1-2) -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -m64 -mno-80387 -mno-fp-ret-in-387 -mpreferred-stack-boundary=3 -mskip-rax-setup -mtune=generic -mno-red-zone -mcmodel=kernel -mindirect-branch=thunk-extern -mindirect-branch-register -mrecord-mcount -mfentry -march=x86-64 -g -O2 -std=gnu90 -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE -falign-jumps=1 -falign-loops=1 -fno-asynchronous-unwind-tables -fno-jump-tables -fno-delete-null-pointer-checks -fstack-protector-strong -fno-var-tracking-assignments -fno-strict-overflow -fno-merge-all-constants -fmerge-constants -fstack-check=no -fconserve-stack -fcf-protection=none --param allow-store-data-races=0
>>>> [acme@five pahole]$
>>>>
>>>> > Thanks,
>>>> >
>>>> > - Arnaldo
>>>> >
>>>> > > Testing:
>>>> > >
>>>> > > Before:
>>>> > >  $ readelf -SW vmlinux | grep BTF
>>>> > >  [25] .BTF              PROGBITS        ffffffff821a905c 13a905c 2d2bf8 00   A  0   0  1
>>>> > >
>>>> > > After:
>>>> > >  $ pahole -J vmlinux
>>>> > >  $ readelf -SW vmlinux  | grep BTF
>>>> > >  [25] .BTF              PROGBITS        ffffffff821a905c 13a905c 2d5bca 00   A  0   0  1
>>>> > >
>>>> > > Common percpu vars can be found in the BTF section.
>>>> > >
>>>> > >  $ bpftool btf dump file vmlinux | grep runqueues
>>>> > >  [14098] VAR 'runqueues' type_id=13725, linkage=global-alloc
>>>> > >
>>>> > >  $ bpftool btf dump file vmlinux | grep 'cpu_stopper'
>>>> > >  [17592] STRUCT 'cpu_stopper' size=72 vlen=5
>>>> > >  [17612] VAR 'cpu_stopper' type_id=17592, linkage=static
>>>> > >
>>>> > >  $ bpftool btf dump file vmlinux | grep ' DATASEC '
>>>> > >  [63652] DATASEC '.data..percpu' size=0 vlen=294
>>>> > >
>>>> > > References:
>>>> > >  [1] https://lwn.net/Articles/531148/
>>>> > > Signed-off-by: Hao Luo <haoluo-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>>>> > > ---
>>>> > > Changelog since v2:
>>>> > > - Move finding percpu_shndx and extracting symtab into btfe creation,
>>>> > >   so we don't have to allocate a new symtab for each CU.
>>>> > > - More debug msg by logging the vars encoded in 'verbose' mode. We
>>>> > >   probably don't want to log the symbols that are _not_ encoded,
>>>> > >   since that would be too verbose.
>>>> > > - Calculate var offsets using 'addr - shdr.sh_addr', so it could be
>>>> > >   generalized to other sections in future.
>>>> > > - Filter out the symbols that are not STT_OBJECT.
>>>> > > - Sort var_secinfos in the DATASEC by their offsets.
>>>> > > - Free 'persec_secinfo' buffer and 'symtab' in btfe deletion.
>>>> > > - Replace the string ".data..percpu" with a constant PERCPU_SECTION.
>>>> > >
>>>> > > Changelog since v1:
>>>> > > - Add a ".data..percpu" DATASEC that encodes the found VARs.
>>>> > > - Use percpu section's shndx to find the symbols that are percpu variables.
>>>> > > - Use the correct type to set VAR's linkage.
>>>> > >
>>>> > >  btf_encoder.c | 119 ++++++++++++++++++++++++++++++++++++++++++++++++
>>>> > >  dwarves.c     |   6 +++
>>>> > >  dwarves.h     |   2 +
>>>> > >  libbtf.c      | 123 ++++++++++++++++++++++++++++++++++++++++++++++++++
>>>> > >  libbtf.h      |  12 +++++
>>>> > >  pahole.c      |   1 +
>>>> > >  6 files changed, 263 insertions(+)
>>>> > >

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