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

[Vineet Gupta]

On 4/29/19 6:18 PM, Arnaldo Carvalho de Melo wrote:
>>> Auto-detecting system features:
>>> ...                         dwarf: [ OFF ]
>>> ...            dwarf_getlocations: [ OFF ]
>>> ...                         glibc: [ on  ]
>> Not related to current issue, this run uses a uClibc toolchain and yet it is
>> detecting glibc - doesn't seem right to me.
> Ok, I'll improve that, I think it just tries to detect a libc, yeah,
> see:
>
> [acme@quaco linux]$ cat tools/build/feature/test-glibc.c
> // SPDX-License-Identifier: GPL-2.0
> #include <stdlib.h>
>
> #if !defined(__UCLIBC__)
> #include <gnu/libc-version.h>
> #else
> #define XSTR(s) STR(s)
> #define STR(s) #s
> #endif
>
> int main(void)
> {
> #if !defined(__UCLIBC__)
> 	const char *version = gnu_get_libc_version();
> #else
> 	const char *version = XSTR(__GLIBC__) "." XSTR(__GLIBC_MINOR__);
> #endif
>
> 	return (long)version;
> }
> [acme@quaco linux]$
>
> [perfbuilder@59ca4b424ded /]$ grep __GLIBC__ /arc_gnu_2017.09-rc2_prebuilt_uclibc_le_arc700_linux_install/arc-snps-linux-uclibc/sysroot/usr/include/*.h
> /arc_gnu_2017.09-rc2_prebuilt_uclibc_le_arc700_linux_install/arc-snps-linux-uclibc/sysroot/usr/include/features.h:   The macros `__GNU_LIBRARY__', `__GLIBC__', and `__GLIBC_MINOR__' are
> /arc_gnu_2017.09-rc2_prebuilt_uclibc_le_arc700_linux_install/arc-snps-linux-uclibc/sysroot/usr/include/features.h:#define	__GLIBC__	2
> /arc_gnu_2017.09-rc2_prebuilt_uclibc_le_arc700_linux_install/arc-snps-linux-uclibc/sysroot/usr/include/features.h:	((__GLIBC__ << 16) + __GLIBC_MINOR__ >= ((maj) << 16) + (min))
> [perfbuilder@59ca4b424ded /]$
>
> Isn't that part of uClibc?

Right you are. Per the big fat comment right above that code, this gross hack in
uclibc is unavoidable as applications tend to rely on that define.
So a better fix would be to check for various !GLIBC libs explicitly.

#ifdef __UCLIBC__

#elseif defined __MUSL__

...

Not pretty from app usage pov, but that seems to be the only sane way of doing it.

-Vineet