linux-kernel - Re: [PATCH v2] selftests/vDSO: fix clang build errors and warnings

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Date: Thu, 30 May 2024 12:24:52 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Muhammad Usama Anjum <usama.anjum@...labora.com>,
 Shuah Khan <shuah@...nel.org>, Andy Lutomirski <luto@...nel.org>
Cc: Mark Brown <broonie@...nel.org>,
 Vincenzo Frascino <vincenzo.frascino@....com>,
 Colin Ian King <colin.i.king@...il.com>,
 Valentin Obst <kernel@...entinobst.de>, linux-kselftest@...r.kernel.org,
 LKML <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH v2] selftests/vDSO: fix clang build errors and warnings

On 5/29/24 1:05 AM, Muhammad Usama Anjum wrote:
> On 5/28/24 2:16 AM, John Hubbard wrote:
>> When building with clang, via:
>>
>>      make LLVM=1 -C tools/testing/selftests
>>
>> ...there are several warnings, and an error. This fixes all of those and
>> allows these tests to run and pass.
>>
>> 1. Fix linker error (undefined reference to memcpy) by providing a local
>>     version of memcpy.
>>
>> 2. clang complains about using this form:
>>
>>      if (g = h & 0xf0000000)
>>
>> ...so factor out the assignment into a separate step.
>>
>> 3. The code is passing a signed const char* to elf_hash(), which expects
>>     a const unsigned char *. There are several callers, so fix this at
>>     the source by allowing the function to accept a signed argument, and
>>     then converting to unsigned operations, once inside the function.
>>
>> 4. clang doesn't have __attribute__((externally_visible)) and generates
>>     a warning to that effect. Fortunately, gcc 12 and gcc 13 do not seem
>>     to require that attribute in order to build, run and pass tests here,
>>     so remove it.
> Just checked with GCC 5.1, it builds fine without any errors.
> 
>>
>> [1] https://lore.kernel.org/all/20240329-selftests-libmk-llvm-rfc-v1-1-2f9ed7d1c49f@valentinobst.de/
>>
>> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> LGTM
> Reviewed-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
> Tested-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
> 

Thanks for the review and testing, Muhammad!

I'm also adding Andy Lutomirski to this thread, in case Shuah would like
the vDSO maintainer to ack or review. (scripts/get_maintainer.pl is letting
me down, I will need to run "git blame" more I guess.

thanks,
-- 
John Hubbard
NVIDIA

>> ---
>>
>> Changes since the first version:
>>
>> 1) Rebased onto Linux 6.10-rc1
>>
>> thanks,
>> John Hubbard
>>
>>   tools/testing/selftests/vDSO/parse_vdso.c      | 16 +++++++++++-----
>>   .../selftests/vDSO/vdso_standalone_test_x86.c  | 18 ++++++++++++++++--
>>   2 files changed, 27 insertions(+), 7 deletions(-)
>>
>> diff --git a/tools/testing/selftests/vDSO/parse_vdso.c b/tools/testing/selftests/vDSO/parse_vdso.c
>> index 413f75620a35..4ae417372e9e 100644
>> --- a/tools/testing/selftests/vDSO/parse_vdso.c
>> +++ b/tools/testing/selftests/vDSO/parse_vdso.c
>> @@ -55,14 +55,20 @@ static struct vdso_info
>>   	ELF(Verdef) *verdef;
>>   } vdso_info;
>>   
>> -/* Straight from the ELF specification. */
>> -static unsigned long elf_hash(const unsigned char *name)
>> +/*
>> + * Straight from the ELF specification...and then tweaked slightly, in order to
>> + * avoid a few clang warnings.
>> + */
>> +static unsigned long elf_hash(const char *name)
>>   {
>>   	unsigned long h = 0, g;
>> -	while (*name)
>> +	const unsigned char *uch_name = (const unsigned char *)name;
>> +
>> +	while (*uch_name)
>>   	{
>> -		h = (h << 4) + *name++;
>> -		if (g = h & 0xf0000000)
>> +		h = (h << 4) + *uch_name++;
>> +		g = h & 0xf0000000;
>> +		if (g)
>>   			h ^= g >> 24;
>>   		h &= ~g;
>>   	}
>> diff --git a/tools/testing/selftests/vDSO/vdso_standalone_test_x86.c b/tools/testing/selftests/vDSO/vdso_standalone_test_x86.c
>> index 8a44ff973ee1..27f6fdf11969 100644
>> --- a/tools/testing/selftests/vDSO/vdso_standalone_test_x86.c
>> +++ b/tools/testing/selftests/vDSO/vdso_standalone_test_x86.c
>> @@ -18,7 +18,7 @@
>>   
>>   #include "parse_vdso.h"
>>   
>> -/* We need a libc functions... */
>> +/* We need some libc functions... */
>>   int strcmp(const char *a, const char *b)
>>   {
>>   	/* This implementation is buggy: it never returns -1. */
>> @@ -34,6 +34,20 @@ int strcmp(const char *a, const char *b)
>>   	return 0;
>>   }
>>   
>> +/*
>> + * The clang build needs this, although gcc does not.
>> + * Stolen from lib/string.c.
>> + */
>> +void *memcpy(void *dest, const void *src, size_t count)
>> +{
>> +	char *tmp = dest;
>> +	const char *s = src;
>> +
>> +	while (count--)
>> +		*tmp++ = *s++;
>> +	return dest;
>> +}
>> +
>>   /* ...and two syscalls.  This is x86-specific. */
>>   static inline long x86_syscall3(long nr, long a0, long a1, long a2)
>>   {
>> @@ -70,7 +84,7 @@ void to_base10(char *lastdig, time_t n)
>>   	}
>>   }
>>   
>> -__attribute__((externally_visible)) void c_main(void **stack)
>> +void c_main(void **stack)
>>   {
>>   	/* Parse the stack */
>>   	long argc = (long)*stack;
>>
>> base-commit: 2bfcfd584ff5ccc8bb7acde19b42570414bf880b
>