x86 compilation has required asm goto support since 4.17.
Since clang does not support asm goto, at 4.17,
Commit b1ae32dbab ("x86/cpufeature: Guard asm_volatile_goto usage
for BPF compilation") worked around the issue by permitting an
alternative implementation without asm goto for clang.
At 5.0, more asm goto usages appeared.
[yhs@148 x86]$ egrep -r asm_volatile_goto
include/asm/cpufeature.h: asm_volatile_goto("1: jmp 6f\n"
include/asm/jump_label.h: asm_volatile_goto("1:"
include/asm/jump_label.h: asm_volatile_goto("1:"
include/asm/rmwcc.h: asm_volatile_goto (fullop "; j" #cc " %l[cc_label]" \
include/asm/uaccess.h: asm_volatile_goto("\n" \
include/asm/uaccess.h: asm_volatile_goto("\n" \
[yhs@148 x86]$
Compiling samples/bpf directories, most bpf programs failed
compilation with error messages like:
In file included from /home/yhs/work/bpf-next/samples/bpf/xdp_sample_pkts_kern.c:2:
In file included from /home/yhs/work/bpf-next/include/linux/ptrace.h:6:
In file included from /home/yhs/work/bpf-next/include/linux/sched.h:15:
In file included from /home/yhs/work/bpf-next/include/linux/sem.h:5:
In file included from /home/yhs/work/bpf-next/include/uapi/linux/sem.h:5:
In file included from /home/yhs/work/bpf-next/include/linux/ipc.h:9:
In file included from /home/yhs/work/bpf-next/include/linux/refcount.h:72:
/home/yhs/work/bpf-next/arch/x86/include/asm/refcount.h:70:9: error: 'asm goto' constructs are not supported yet
return GEN_BINARY_SUFFIXED_RMWcc(LOCK_PREFIX "subl",
^
/home/yhs/work/bpf-next/arch/x86/include/asm/rmwcc.h:67:2: note: expanded from macro 'GEN_BINARY_SUFFIXED_RMWcc'
__GEN_RMWcc(op " %[val], %[var]\n\t" suffix, var, cc, \
^
/home/yhs/work/bpf-next/arch/x86/include/asm/rmwcc.h:21:2: note: expanded from macro '__GEN_RMWcc'
asm_volatile_goto (fullop "; j" #cc " %l[cc_label]" \
^
/home/yhs/work/bpf-next/include/linux/compiler_types.h:188:37: note: expanded from macro 'asm_volatile_goto'
#define asm_volatile_goto(x...) asm goto(x)
Most implementation does not even provide an alternative
implementation. And it is also not practical to make changes
for each call site.
This patch workarounded the asm goto issue by redefining the macro like below:
#define asm_volatile_goto(x...) asm volatile("invalid use of asm_volatile_goto")
If asm_volatile_goto is not used by bpf programs, which is typically the case, nothing bad
will happen. If asm_volatile_goto is used by bpf programs, which is incorrect, the compiler
will issue an error since "invalid use of asm_volatile_goto" is not valid assembly codes.
With this patch, all bpf programs under samples/bpf can pass compilation.
Note that bpf programs under tools/testing/selftests/bpf/ compiled fine as
they do not access kernel internal headers.
Fixes: e769742d35 ("Revert "x86/jump-labels: Macrofy inline assembly code to work around GCC inlining bugs"")
Fixes: 18fe58229d ("x86, asm: change the GEN_*_RMWcc() macros to not quote the condition")
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
eBPF sample programs
====================
This directory contains a test stubs, verifier test-suite and examples
for using eBPF. The examples use libbpf from tools/lib/bpf.
Build dependencies
==================
Compiling requires having installed:
* clang >= version 3.4.0
* llvm >= version 3.7.1
Note that LLVM's tool 'llc' must support target 'bpf', list version
and supported targets with command: ``llc --version``
Kernel headers
--------------
There are usually dependencies to header files of the current kernel.
To avoid installing devel kernel headers system wide, as a normal
user, simply call::
make headers_install
This will creates a local "usr/include" directory in the git/build top
level directory, that the make system automatically pickup first.
Compiling
=========
For building the BPF samples, issue the below command from the kernel
top level directory::
make samples/bpf/
Do notice the "/" slash after the directory name.
It is also possible to call make from this directory. This will just
hide the the invocation of make as above with the appended "/".
Manually compiling LLVM with 'bpf' support
------------------------------------------
Since version 3.7.0, LLVM adds a proper LLVM backend target for the
BPF bytecode architecture.
By default llvm will build all non-experimental backends including bpf.
To generate a smaller llc binary one can use::
-DLLVM_TARGETS_TO_BUILD="BPF"
Quick sniplet for manually compiling LLVM and clang
(build dependencies are cmake and gcc-c++)::
$ git clone http://llvm.org/git/llvm.git
$ cd llvm/tools
$ git clone --depth 1 http://llvm.org/git/clang.git
$ cd ..; mkdir build; cd build
$ cmake .. -DLLVM_TARGETS_TO_BUILD="BPF;X86"
$ make -j $(getconf _NPROCESSORS_ONLN)
It is also possible to point make to the newly compiled 'llc' or
'clang' command via redefining LLC or CLANG on the make command line::
make samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
Cross compiling samples
-----------------------
In order to cross-compile, say for arm64 targets, export CROSS_COMPILE and ARCH
environment variables before calling make. This will direct make to build
samples for the cross target.
export ARCH=arm64
export CROSS_COMPILE="aarch64-linux-gnu-"
make samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
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