netdev - Re: [PATCH bpf] samples/bpf: workaround clang asm goto compilation errors

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Message-ID: <8d793502-f7c9-d007-86bc-7cc7be7044da@iogearbox.net>
Date:   Tue, 15 Jan 2019 21:02:02 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Yonghong Song <yhs@...com>, ast@...com, netdev@...r.kernel.org
Cc:     kernel-team@...com
Subject: Re: [PATCH bpf] samples/bpf: workaround clang asm goto compilation
 errors

On 01/13/2019 12:04 AM, Yonghong Song wrote:
> x86 compilation has required asm goto support since 4.17.
> Since clang does not support asm goto, at 4.17,
> Commit b1ae32dbab50 ("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: e769742d3584 ("Revert "x86/jump-labels: Macrofy inline assembly code to work around GCC inlining bugs"")
> Fixes: 18fe58229d80 ("x86, asm: change the GEN_*_RMWcc() macros to not quote the condition")
> Acked-by: Alexei Starovoitov <ast@...nel.org>
> Signed-off-by: Yonghong Song <yhs@...com>

Applied, thanks!