On Wed, Jul 8, 2020 at 1:44 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> 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 v5.8-rc3 linux kernel, I was able to extract 288 such variables.
> The build time overhead is small and the space overhead is also small.
> See testings below.
>
> A found variable can be invalid in two ways:
>
> - Its name found in elf_sym__name is invalid.
> - Its size identified by elf_sym__size is 0.
>
> In either case, the btf containing such symbols will be rejected by the
> btf verifier. Normally we should not see such symbols. But if one is
> seen during btf encoding, the encoder will exit with error. An new flag
> '-j' (or '--force') is implemented to help testing, which skips the
> invalid symbols and force emit a btf.
>
> Testing:
>
> - Vmlinux size has increased by ~12kb.
> Before:
> $ readelf -SW vmlinux | grep BTF
> [25] .BTF PROGBITS ffffffff821a905c 13a905c 2d2bf8 00
> After:
> $ pahole -J vmlinux
> $ readelf -SW vmlinux | grep BTF
> [25] .BTF PROGBITS ffffffff821a905c 13a905c 2d5bca 00
>
> - Common global percpu VARs and DATASEC are found in BTF section.
> $ bpftool btf dump file vmlinux | grep runqueues
> [14152] VAR 'runqueues' type_id=13778, linkage=global-alloc
>
> $ bpftool btf dump file vmlinux | grep 'cpu_stopper'
> [17582] STRUCT 'cpu_stopper' size=72 vlen=5
> [17601] VAR 'cpu_stopper' type_id=17582, linkage=static
>
> $ bpftool btf dump file vmlinux | grep ' DATASEC '
> [63652] DATASEC '.data..percpu' size=179288 vlen=288
>
> - Tested bpf selftests.
>
> - Pahole exits with error if an invalid symbol is seen during encoding,
> make -f Makefile -j 36 -s
> PAHOLE: Error: Found symbol of zero size when encoding btf (sym: 'yyy', cu: 'xxx.c').
> PAHOLE: Error: Use '-j' or '--force_emit' to ignore such symbols and force emit the btf.
> scripts/link-vmlinux.sh: line 137: 2475712 Segmentation fault LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
>
> - With the flag '-j' or '--force', the invalid symbols are ignored.
>
> - Further in verbose mode and with '-j' or '--force' set, a warning is generated:
> PAHOLE: Warning: Found symbol of zero size when encoding btf, ignored (sym: 'yyy', cu: 'xxx.c').
> PAHOLE: Warning: Found symbol of invalid name when encoding btf, ignored (sym: 'zzz', cu: 'sss.c').
>
> References:
> [1] https://lwn.net/Articles/531148/
>
> Acked-by: Andrii Nakryiko <andriin@xxxxxx>
> Tested-by: Andrii Nakryiko <andriin@xxxxxx>
> Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> ---
Looks great, thanks!
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