linux-kernel - Re: [PATCH 1/3] lib: early_string: allow early usage of some string functions

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Message-ID: <1929b3a8-f882-c930-4b99-10c6a8f127c7@csgroup.eu>
Date:   Fri, 30 Apr 2021 10:50:39 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Daniel Walker <danielwa@...co.com>, linuxppc-dev@...ts.ozlabs.org,
        x86@...nel.org, Andrew Morton <akpm@...ux-foundation.org>
Cc:     xe-linux-external@...co.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] lib: early_string: allow early usage of some string
 functions



Le 30/04/2021 à 10:47, Christophe Leroy a écrit :
> 
> 
> Le 30/04/2021 à 06:22, Daniel Walker a écrit :
>> This systems allows some string functions to be moved into
>> lib/early_string.c and they will be prepended with "early_" and compiled
>> without debugging like KASAN.
>>
>> This is already done on x86 for,
>> "AMD Secure Memory Encryption (SME) support"
>>
>> and on powerpc prom_init.c , and EFI's libstub.
>>
>> The AMD memory feature disabled KASAN for all string functions, and
>> prom_init.c and efi libstub implement their own versions of the
>> functions.
>>
>> This implementation allows sharing of the string functions without
>> removing the debugging features for the whole system.
> 
> This looks good. I prefer that rather than the way you proposed to do it two years ago.
> 
> Only one problem, see below.
> 
>> +size_t strlcat(char *dest, const char *src, size_t count)
>> +{
>> +    size_t dsize = strlen(dest);
>> +    size_t len = strlen(src);
>> +    size_t res = dsize + len;
>> +
>> +    /* This would be a bug */
>> +    BUG_ON(dsize >= count);
> 
> powerpc is not ready to handle BUG_ON() in when in prom_init.
> 
> Can you do:
> 
> #ifndef __EARLY_STRING_ENABLED
>      BUG_ON(dsize >= count);
> #endif

In fact, should be like in prom_init today:

#ifdef __EARLY_STRING_ENABLED
	if (dsize >= count)
		return count;
#else
	BUG_ON(dsize >= count);
#endif

> 
> 
>> +
>> +    dest += dsize;
>> +    count -= dsize;
>> +    if (len >= count)
>> +        len = count-1;
>> +    memcpy(dest, src, len);
>> +    dest[len] = 0;
>> +    return res;
>> +}
>> +EXPORT_SYMBOL(strlcat);
>> +#endif
>> +