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Subject: Re: [PATCH 1/3] lib: early_string: allow early usage of some string
 functions
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
To:     Daniel Walker <danielwa@cisco.com>, linuxppc-dev@lists.ozlabs.org,
        x86@kernel.org, Andrew Morton <akpm@linux-foundation.org>
Cc:     xe-linux-external@cisco.com, linux-kernel@vger.kernel.org
References: <20210430042217.1198052-1-danielwa@cisco.com>
 <dc26a67e-dba0-1b8c-3718-3c75415c61f1@csgroup.eu>
 <1929b3a8-f882-c930-4b99-10c6a8f127c7@csgroup.eu>
Message-ID: <e355ecc9-574a-dbcb-7864-5aa4974e1971@csgroup.eu>
Date:   Sat, 1 May 2021 09:31:47 +0200
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Le 30/04/2021 à 10:50, Christophe Leroy a écrit :
> 
> 
> 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

Thinking about it once more, this BUG_ON() is overkill and should be avoided, see 
https://www.kernel.org/doc/html/latest/process/deprecated.html

Therefore, something like the following would make it:

	if (dsize >= count) {
		WARN_ON(!__is_defined(__EARLY_STRING_ENABLED));

		return count;
	}

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