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

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Message-ID: <CAEf4BzbhH9DtBsen8x9BUrAZy0d-osK57Zg=Cor-BmLyvAX+jg@mail.gmail.com>
Date:   Sat, 27 Aug 2022 10:08:37 -0700
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 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:

Acked-by: Andrii Nakryiko <andrii@...nel.org>

> >
> > 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
> > >