Redefine asm_inline for Linux 5.4+, Fixes #2546 #2547
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Here's the upstream commit that introduced `asm_inline`: * torvalds/linux@eb11186 Without this patch BCC fails with the following: ``` $ sudo /usr/share/bcc/tools/opensnoop In file included from /virtual/main.c:2: In file included from include/uapi/linux/ptrace.h:142: In file included from ./arch/x86/include/asm/ptrace.h:5: ./arch/x86/include/asm/segment.h:254:2: error: expected '(' after 'asm' alternative_io ("lsl %[seg],%[p]", ^ ./arch/x86/include/asm/alternative.h:240:2: note: expanded from macro 'alternative_io' asm_inline volatile (ALTERNATIVE(oldinstr, newinstr, feature) \ ^ include/linux/compiler_types.h:210:24: note: expanded from macro 'asm_inline' ^ ```
bobrik
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yonghong-song
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danobi
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Apr 17, 2020
BPF Type Format (BTF) is especially powerful for tracing tools like
bcc and bpftrace. It allows tools to know about kernel data structures
and layouts without having to parse headers. Headers are problematic
because (1) they do not always come installed on production / user
systems, (2) headers may not always describe the correct struct layout
due to compile time flags, and (3) can be tricky to parse [0][1].
As BTF becomes enabled on more systems [2], it becomes more desirable to
have BTF contain everything a tracing tool needs. BTF overhead is quite
minimal (~1.5MB) [3] so using BTF in lieu of parsing headers is very
attractive.
While BTF already contains almost everything tracing tools need (eg
structs, enums, unions, function signatures, etc.), it is missing a lot
of flags. The reason is that most flags are defined as preprocessor
macros and are thus invisible to the compiler when it generates debug
info.
However, there is a solution: we can convert macro flags into enums.
This would be quite a complicated and long running task so I'm hoping
this patch can start a discussion. I'm sure I haven't fully considered
the implications so hopefully we can discuss it.
This patch, when applied to a kernel with BTF, allows bpftrace to "see"
the flags [4]:
# bpftrace --btf -e 'BEGIN { printf("%d\n", S_IRWXG); }'
Attaching 1 probe...
56
^C
[0]: iovisor/bcc#2133
[1]: iovisor/bcc#2547
[2]: https://www.spinics.net/linux/fedora/fedora-kernel/msg07746.html
[3]: https://facebookmicrosites.github.io/bpf/blog/2018/11/14/btf-enhancement.html
[4]: bpftrace/bpftrace#1274
CrackerCat
pushed a commit
to CrackerCat/bcc
that referenced
this pull request
Jul 31, 2024
Here's the upstream commit that introduced `asm_inline`: * torvalds/linux@eb11186 Without this patch BCC fails with the following: ``` $ sudo /usr/share/bcc/tools/opensnoop In file included from /virtual/main.c:2: In file included from include/uapi/linux/ptrace.h:142: In file included from ./arch/x86/include/asm/ptrace.h:5: ./arch/x86/include/asm/segment.h:254:2: error: expected '(' after 'asm' alternative_io ("lsl %[seg],%[p]", ^ ./arch/x86/include/asm/alternative.h:240:2: note: expanded from macro 'alternative_io' asm_inline volatile (ALTERNATIVE(oldinstr, newinstr, feature) \ ^ include/linux/compiler_types.h:210:24: note: expanded from macro 'asm_inline' ^ ```
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Here's the upstream commit that introduced
asm_inline:Without this patch BCC fails with the following: