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

[Vineet Gupta]

On 4/30/19 10:04 AM, Rich Felker wrote:
> On Tue, Apr 30, 2019 at 03:53:18PM +0000, Vineet Gupta wrote:
>> 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.
> 
> 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