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

[Christophe Leroy]

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
> > > +