On Thu, Aug 10, 2017 at 12:21:04PM +0200, Lennart Poettering wrote: > On Do, 10.08.17 13:25, NeilBrown (neilb@xxxxxxxx) wrote: > > +.RB ( ".f_type == AUTOFS_SUPER_MAGIC" ). > > Because Linux is broken you shouldn't compare f_type just like this, > and the man page probably shouldn't suggest that either I figure. The > only safe way is something like this: > > s.f_type == (typeof(s.f_type)) AUTOFS_SUPER_MAGIC > > That's because f_type is defined with different types (both signed and > unsigned) on different archs, and the magic values tend to use the > full unsigned 32bit range... Please give an example where Neil's formulation would fail. $ git grep -w f_type arch/*/include |cut -d: -f 2- |sort -u int f_type; long f_type; s32 f_type; __u32 f_type; u32 f_type; unsigned int f_type; I quote the C standard here for your convenience: Otherwise, the integer promotions are performed on both operands. Then the following rules are applied to the promoted operands: If both operands have the same type, then no further conversion is needed. Otherwise, if both operands have signed integer types or both have unsigned integer types, the operand with the type of lesser integer conversion rank is converted to the type of the operand with greater rank. Otherwise, if the operand that has unsigned integer type has rank greater or equal to the rank of the type of the other operand, then the operand with signed integer type is converted to the type of the operand with unsigned integer type. Otherwise, if the type of the operand with signed integer type can represent all of the values of the type of the operand with unsigned integer type, then the operand with unsigned integer type is converted to the type of the operand with signed integer type. Otherwise, both operands are converted to the unsigned integer type corresponding to the type of the operand with signed integer type > (Yes, strictly speaking AUTOFS_SUPER_MAGIC isn't one of the unsigned > 32bit ones, but I think it's better to stick the same rules for all > magic values comparisons here...) > > (And yes, the statfs() man page only mentions the problem briefly, > without the typeof way out, but it really should) > > Lennart > > -- > Lennart Poettering, Red Hat -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
