Detecting libc in perf (was Re: perf tools build broken after v5.1-rc1)

[acme@kernel.org (Arnaldo Carvalho de Melo)]

Em Thu, May 02, 2019 at 09:55:26AM -0700, Vineet Gupta escreveu:
> On 4/30/19 8:12 PM, Rich Felker wrote:
> >>> What are you trying to achieve? I was just CC'd and I'm missing the
> >>> context.
> >>
> >> Sorry I added you as a subject matter expert but didn't provide enough context.
> >>
> >> The original issue [1] was perf failing to build on ARC due to perf tools needing
> >> a copy of unistd.h but this thread [2] was a small side issue of auto-detecting
> >> libc variaint in perf tools where despite uClibc tools, glibc is declared to be
> >> detected, due to uClibc's historical hack of defining __GLIBC__. So __GLIBC__ is
> >> not sufficient (and probably not the right interface to begin wtih) to ensure glibc.
> >>
> >> [1] http://lists.infradead.org/pipermail/linux-snps-arc/2019-April/005676.html
> >> [2] http://lists.infradead.org/pipermail/linux-snps-arc/2019-April/005684.html
> > 
> > I think you misunderstood -- 
> 
> :-)
> 
> > I'm asking what you're trying to achieve
> > by detecting whether the libc is glibc, rather than whether it has
> > some particular interface you want to conditionally use. This is a
> > major smell and is usually something wrong that shouldn't be done.
> 
> Good question indeed. Back in 2015 I initially ran into some quirks due to subtle
> libc differences.  At the time perf has a fwd ref for strlcpy which exactly
> matched glibc but not uClibc.  see commit  a83d869f300bf91 "(perf tools: Elide
> strlcpy warning with uclibc)" or 0215d59b154 "(tools lib: Reinstate strlcpy()
> header guard with __UCLIBC__)"
> 
> But this still used the libc defined symbol __UCLIBC__ or __GLIBC__
> 
> Your question however pertains to perf glibc feature check where perf generates an
> alternate symbol HAVE_GLIBC_SUPPORT.
> 
> This is dubious as first of all it detects glibc even for uClibc builds.

 
> Even of we were to improve it, there seems to be no users of this symbol.
> 
> $git grep HAVE_GLIBC_SUPPORT
> perf/Makefile.config:  CFLAGS += -DHAVE_GLIBC_SUPPORT
> perf/builtin-version.c: STATUS(HAVE_GLIBC_SUPPORT, glibc)
> 
> So I'd propose to remove it !

This is some remnant of the past, I'll check further but will end up
just ditching it altogether as you suggest :-)

[acme at quaco perf]$ find tools/ -type f | xargs grep HAVE_GLIBC_SUPPORT
tools/perf/builtin-version.c:	STATUS(HAVE_GLIBC_SUPPORT, glibc);
tools/perf/Makefile.config:  CFLAGS += -DHAVE_GLIBC_SUPPORT
[acme at quaco perf]$

Its just this case that ends up using that feature detection program,

[acme at quaco perf]$ vim tools/perf/Makefile.config
[acme at quaco perf]$ find tools/ -type f | xargs grep feature-glibc
tools/perf/Makefile.config:    ifeq ($(feature-glibc), 1)
tools/perf/Makefile.config:ifeq ($(feature-glibc), 1)
[acme at quaco perf]$

BTW the function on it doesn't mean anything, what matters is if the
program builds or not :-)

- Arnaldo