linux-kernel - Re: [PATCH] selftests/bpf: Fix conflicts with built-in functions in bpf_iter

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Date:   Fri, 2 Dec 2022 16:53:44 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     James Hilliard <james.hilliard1@...il.com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
        Alan Maguire <alan.maguire@...cle.com>,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftests/bpf: Fix conflicts with built-in functions in bpf_iter_ksym

On Fri, Dec 2, 2022 at 4:41 PM James Hilliard <james.hilliard1@...il.com> wrote:
>
> On Sat, Aug 27, 2022 at 1:08 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Sat, Aug 27, 2022 at 3:47 AM James Hilliard
> > <james.hilliard1@...il.com> wrote:
> > >
> > > On Fri, Aug 26, 2022 at 10:55 PM Andrii Nakryiko
> > > <andrii.nakryiko@...il.com> wrote:
> > > >
> > > > On Thu, Aug 25, 2022 at 6:26 PM James Hilliard
> > > > <james.hilliard1@...il.com> wrote:
> > > > >
> > > > > Both tolower and toupper are built in c functions, we should not
> > > >
> > > > Really? How come? Can you point out where this is specified in C
> > > > standard? From what I can tell you have to include <ctype.h> to get
> > > > toupper()/tolower().
> > >
> > > See background on this sort of issue:
> > > https://stackoverflow.com/a/20582607
> > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12213
> > >
> > > (C99, 7.1.3p1) "All identifiers with external linkage in any of the following
> > > subclauses (including the future library directions) are always reserved
> > > for use as identifiers with external linkage."
> > >
> >
> > Sigh, ok. Ghost functions: you can't use them without ctype.h, but you
> > can't define your own either. Very nice.
> >
> > Can you please put all these details into a commit for the future? With that:
>
> Sure
>
> >
> > Acked-by: Andrii Nakryiko <andrii@...nel.org>
>
> Is this good to apply or should I send a v2?

please add those details and resend v2?

>
> >
> > > >
> > > > This seems like yet another GCC-BPF quirk?
> > >
> > > Seems GCC takes a stricter interpretation of the standard here than
> > > llvm.
> > >
> > > It's also documented behavior in GCC:
> > > https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-std-2
> > >
> > > See:
> > > The ISO C90 functions abort, abs, acos, asin, atan2, atan, calloc, ceil,
> > > cosh, cos, exit, exp, fabs, floor, fmod, fprintf, fputs, free, frexp, fscanf,
> > > isalnum, isalpha, iscntrl, isdigit, isgraph, islower, isprint, ispunct, isspace,
> > > isupper, isxdigit, tolower, toupper, labs, ldexp, log10, log, malloc, memchr,
> > > memcmp, memcpy, memset, modf, pow, printf, putchar, puts, realloc,
> > > scanf, sinh, sin, snprintf, sprintf, sqrt, sscanf, strcat, strchr,
> > > strcmp, strcpy,
> > > strcspn, strlen, strncat, strncmp, strncpy, strpbrk, strrchr, strspn, strstr,
> > > tanh, tan, vfprintf, vprintf and vsprintf are all recognized as
> > > built-in functions
> > > unless -fno-builtin is specified (or -fno-builtin-function is specified for an
> > > individual function).
> > >
> > > We could disable builtin functions but it seems more correct to just
> > > rename it so that it doesn't conflict.
> > >
> > > >
> > > > > redefine them as this can result in a build error.
> > > > >
> > > > > Fixes the following errors:
> > > > > progs/bpf_iter_ksym.c:10:20: error: conflicting types for built-in function 'tolower'; expected 'int(int)' [-Werror=builtin-declaration-mismatch]
> > > > >    10 | static inline char tolower(char c)
> > > > >       |                    ^~~~~~~
> > > > > progs/bpf_iter_ksym.c:5:1: note: 'tolower' is declared in header '<ctype.h>'
> > > > >     4 | #include <bpf/bpf_helpers.h>
> > > > >   +++ |+#include <ctype.h>
> > > > >     5 |
> > > > > progs/bpf_iter_ksym.c:17:20: error: conflicting types for built-in function 'toupper'; expected 'int(int)' [-Werror=builtin-declaration-mismatch]
> > > > >    17 | static inline char toupper(char c)
> > > > >       |                    ^~~~~~~
> > > > > progs/bpf_iter_ksym.c:17:20: note: 'toupper' is declared in header '<ctype.h>'
> > > > >
> > > > > Signed-off-by: James Hilliard <james.hilliard1@...il.com>
> > > > > ---
> > > > >  tools/testing/selftests/bpf/progs/bpf_iter_ksym.c | 6 +++---
> > > > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > > > >
> > > > > diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_ksym.c b/tools/testing/selftests/bpf/progs/bpf_iter_ksym.c
> > > > > index 285c008cbf9c..9ba14c37bbcc 100644
> > > > > --- a/tools/testing/selftests/bpf/progs/bpf_iter_ksym.c
> > > > > +++ b/tools/testing/selftests/bpf/progs/bpf_iter_ksym.c
> > > > > @@ -7,14 +7,14 @@ char _license[] SEC("license") = "GPL";
> > > > >
> > > > >  unsigned long last_sym_value = 0;
> > > > >
> > > > > -static inline char tolower(char c)
> > > > > +static inline char to_lower(char c)
> > > > >  {
> > > > >         if (c >= 'A' && c <= 'Z')
> > > > >                 c += ('a' - 'A');
> > > > >         return c;
> > > > >  }
> > > > >
> > > > > -static inline char toupper(char c)
> > > > > +static inline char to_upper(char c)
> > > > >  {
> > > > >         if (c >= 'a' && c <= 'z')
> > > > >                 c -= ('a' - 'A');
> > > > > @@ -54,7 +54,7 @@ int dump_ksym(struct bpf_iter__ksym *ctx)
> > > > >         type = iter->type;
> > > > >
> > > > >         if (iter->module_name[0]) {
> > > > > -               type = iter->exported ? toupper(type) : tolower(type);
> > > > > +               type = iter->exported ? to_upper(type) : to_lower(type);
> > > > >                 BPF_SEQ_PRINTF(seq, "0x%llx %c %s [ %s ] ",
> > > > >                                value, type, iter->name, iter->module_name);
> > > > >         } else {
> > > > > --
> > > > > 2.34.1
> > > > >