On Fri, Sep 11 2015, Viresh Kumar <viresh.ku...@linaro.org> wrote: > Long back 'bool' type used to be a typecast to 'int', but that changed > in v2.6.19. And that is a typecast to _Bool now, which (mostly) takes > just a byte. Anyway, the bool type in kernel is used to store true/false > or 1/0 only. So, accessing a single byte should be enough. > > The problem with current code is that it reads/writes 4 bytes for a > boolean, which will read/update 3 excess bytes following the boolean > variable. And that can lead to hard to fix bugs. It was a nightmare to > crack this one. > > The debugfs code had this bug since the first time it got introduced, > but was never got caught, strange. Maybe the bool variables (monitored > by debugfs) were followed by an 'int' or something bigger and the pad > bytes made sure, we never see this issue. > > But the OPP (Operating performance points) library have three booleans > allocated to contiguous bytes and this bug got hit quite soon (The > debugfs support for OPP is yet to be merged). > > Fix this by changing type of 'val' pointer to u8 type, so that we only > access a single byte.
If the pointed-to type is supposed to be a bool aka _Bool, shouldn't you cast to bool* instead of assuming sizeof(bool)==1? It's probably non-existing, but imagine a big-endian architecture where sizeof(bool)==4; you'd end up reading/writing the wrong byte. > Also, there is another problem I see, which probably should be fixed as > well. But I wanted to hear from you before trying to patch the kernel > for this. > > debugfs_create_bool() declares the pointer to be of type u32 *. > Shouldn't that be changed to u8 *? There are many users which are > typecasting the variables to make debugfs API happy :) Hm, yes, that's annoying. But since most people currently do pass an u32, treating the pointer as u8* is wrong on big-endian (though of course it doesn't matter if the value is only ever checked for being zero/non-zero). So it would probably be better to change the debugfs_create_bool to actually expect a bool* - there aren't _that_ many current callers, and some are obviously aware of the weirdness (with comments such as 'must be u32 for debugfs_create_bool'). Rasmus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
