Re: [PATCH bpf-next v5 00/10] BTF: BPF Type Format

[Martin KaFai Lau <kafai@fb.com>]

On Thu, Jun 14, 2018 at 12:03:34PM -0300, Arnaldo Carvalho de Melo wrote:

> > > > > 1. The tools/testing/selftests/bpf/Makefile has the CLANG_FLAGS and
> > > > >    LLC_FLAGS needed to compile the bpf prog.  It requires a new
> > > > >    "-mattr=dwarf" llc option which was added to the future
> > > > >    llvm 7.0.

[ ... ]

> I tried it, but it didn't work, see:
> 
> [root@jouet bpf]# cat hello.c 
> #include "stdio.h"
> 
> int syscall_enter(openat)(void *ctx)
> {
> 	puts("Hello, world\n");
> 	return 0;
> }
> [root@jouet bpf]# trace -e openat,hello.c touch /tmp/kafai
> clang-6.0: error: unknown argument: '-mattr=dwarf'
"-mattr=dwarf" is currently a llc only option.

tools/testing/selftests/bpf/Makefile has example on how to pipe clang to llc.

e.g.:
clang -g -O2 -target bpf -emit-llvm -c hello.c -o - | llc -march=bpf -mcpu=generic -mattr=dwarfris -filetype=obj -o hello.o

> ERROR:	unable to compile hello.c
> Hint:	Check error message shown above.
> Hint:	You can also pre-compile it into .o using:
>      		clang -target bpf -O2 -c hello.c
>      	with proper -I and -D options.
> event syntax error: 'hello.c'
>                      \___ Failed to load hello.c from source: Error when compiling BPF scriptlet
> 
> (add -v to see detail)
> Run 'perf list' for a list of valid events
> 
>  Usage: perf trace [<options>] [<command>]
>     or: perf trace [<options>] -- <command> [<options>]
>     or: perf trace record [<options>] [<command>]
>     or: perf trace record [<options>] -- <command> [<options>]
> 
>     -e, --event <event>   event/syscall selector. use 'perf list' to list available events
> [root@jouet bpf]#
> 
> The full command line with that is:
> 
> [root@jouet bpf]# trace -v -e openat,hello.c touch /tmp/kafai |& grep mattr
> set env: CLANG_OPTIONS=-g -mattr=dwarf
> llvm compiling command : /usr/local/bin/clang -D__KERNEL__ -D__NR_CPUS__=4 -DLINUX_VERSION_CODE=0x41100 -g -mattr=dwarf  -nostdinc -isystem /usr/lib/gcc/x86_64-redhat-linux/7/include -I/home/acme/git/linux/arch/x86/include -I./arch/x86/include/generated  -I/home/acme/git/linux/include -I./include -I/home/acme/git/linux/arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I/home/acme/git/linux/include/uapi -I./include/generated/uapi -include /home/acme/git/linux/include/linux/kconfig.h  -I/home/acme/lib/include/perf/bpf -Wno-unused-value -Wno-pointer-sign -working-directory /lib/modules/4.17.0-rc5/build -c /home/acme/bpf/hello.c -target bpf -O2 -o -
> clang-6.0: error: unknown argument: '-mattr=dwarf'
> [root@jouet bpf]#
> 
> This is with these llvm and clang trees:
> 
> [root@jouet llvm]# git log --oneline -5
> 98c78e82f54 (HEAD -> master, origin/master, origin/HEAD) [asan] Instrument comdat globals on COFF targets
> 6ad988b5998 [DAGCombiner] clean up comments; NFC
> a735ba5b795 [X86][SSE] Support v8i16/v16i16 rotations
> 1503b9f6fe8 [x86] add tests for node-level FMF; NFC
> 4a49826736f [x86] regenerate test checks; NFC
> [root@jouet llvm]#
> 
> [root@jouet llvm]# cd tools/clang/
> [root@jouet clang]# git log --oneline -5
> 8c873daccc (HEAD -> master, origin/master, origin/HEAD) [X86] Add builtins for vpermq/vpermpd instructions to enable target feature checking.
> a344be6ba4 [X86] Change immediate type for some builtins from char to int.
> dcdd53793e [CUDA] Fix emission of constant strings in sections
> a90c85acaf [X86] Add builtins for shufps and shufpd to enable target feature and immediate range checking.
> ff71c0eccc [X86] Add builtins for pshufd, pshuflw, and pshufhw to enable target feature and immediate range checking.
> [root@jouet clang]#
> 
> [root@jouet clang]# git log | grep mattr=dwarf
> [root@jouet clang]# cd -
> /home/acme/git.tmp/git/llvm
> [root@jouet llvm]# git log | grep mattr=dwarf
>     bpf: introduce -mattr=dwarfris to disable DwarfUsesRelocationsAcrossSections
>     This patch introduces a new flag -mattr=dwarfris
> [root@jouet llvm]#
> 
> Humm, so its -mattr=dwarfris and not -attr=dwarf?
> 
> Didn't help :-\
> 
> commit 0e0047f8c9ada2f0fe0c5f01579a80e2455b8df5
> Author: Yonghong Song <yhs@fb.com>
> Date:   Thu Mar 1 23:04:59 2018 +0000
> 
>     bpf: introduce -mattr=dwarfris to disable DwarfUsesRelocationsAcrossSections
>     
>     Commit e4507fb8c94b ("bpf: disable DwarfUsesRelocationsAcrossSections")
>     disables MCAsmInfo DwarfUsesRelocationsAcrossSections unconditionally
>     so that dwarf will not use cross section (between dwarf and symbol table)
>     relocations. This new debug format enables pahole to dump structures
>     correctly as libdwarves.so does not have BPF backend support yet.
>     
>     This new debug format, however, breaks bcc (https://github.com/iovisor/bcc)
>     source debug output as llvm in-memory Dwarf support has some issues to
>     handle it. More specifically, with DwarfUsesRelocationsAcrossSections
>     disabled, JIT compiler does not generate .debug_abbrev and Dwarf
>     DIE (debug info entry) processing is not happy about this.
>     
>     This patch introduces a new flag -mattr=dwarfris
>     (dwarf relocation in section) to disable DwarfUsesRelocationsAcrossSections.
>     DwarfUsesRelocationsAcrossSections is true by default.
>     
>     Signed-off-by: Yonghong Song <yhs@fb.com>
>     
>     git-svn-id: https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_svn_llvm-2Dproject_llvm_trunk-40326505&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=VQnoQ7LvghIj0gVEaiQSUw&m=LO28-RE-2ZJTXto_gff4BgnxXkbUq8d2CEz1jD_wDl4&s=VCR3pGVfY54-OsZ3BqRsOr3FF5JVyltwbnbzu30_4EY&e= 91177308-0d34-0410-b5e6-96231b3b80d8
> 
>