On Wed, Jul 11, 2012 at 10:22:45PM +0100, Gordan Bobic wrote: > >I will observe that being able to derefrence unaligned pointers is > >something that Linus has said any architecture which does *not* > >provide fixups is probably doomed to fail. > > That sounds like a resigned recognition that a lot of developers > will use unsafe practices anyway, not a model for how things should > be done. The reality is there is a lot of userspace code which does things a certain way. Ignoring that reality will very often doom you to irrelvance. (For example, Plan9 and Mach assumed that the existing huge amount of Posix-compatible userspace code wasn't important, and people would implement new applications to their new OS interfaces from scratch.) > >Yes, someone can send me patches. :-) > > Sure, I can sed __attribute__((aligned(4))) on every char[] > definition, as long as no art rather than engineering oriented > individuals aren;t going to complain that it "looks ugly". :) Um, no. NAK. #1, it's not going to be necessary in all cases, and #2, we try very hard in e2fsprogs to make sure we are not dependent on GCC extensions. If you must use a GCC extension, it must be under an #ifdef __GNUC__ and there must be a reasonable alternative if it's compiled using (for example) the Solaris icc.... Weren't you the one ranting on and on about how portability was so important? > >> I think Gordan suggested (if I understand it > >>right) that doing an array of ints might also solve the problem, since > >>ints should be on natural alignment. Or maybe in some cases malloc/free > >>would be more obvious, if handling errors isn't too tricky. > > > >In the specific case which Gordon has pointed out, the obvious thing > >to do is to just to set errno to ENOMEM, and return -1. since we > >already reflect an error code up to the caller if the FIEMAP ioctl() > >fails. > > > >If someone sends me the patch, I will happily apply it. > > As opposed to aligning the char arrays explicitly? The right thing in the case of the filefrag example which you gave is to allocate the structure dynamically using malloc(). In other cases there may be other solutions which may be a better way to do things. Please do *not* bother to do a global sed style patch. I will reject that summarily. Thought and care are required, not a blunt instrument. > >What we are currently doing may not be 100% portable, but we're not > >going to penalize sane archiectures like x86 just because there might > >be some future insane architecture that requires 32-byte aligned ints! > >All the ranting in the world about how this could hypothetically cause > >file system corruption on said insane architecture will just cause me > >to laugh at you. > > By that definition, the majority of architectures supported by Linux > are insane. Word alignment requirements apply to ARM, and IIRC to > Itanium and SPARC (not sure about the most recent SPARCs) too. If > you think that Linux should become x86-only, that's fair enough, I'm > sure the ARM kernel maintainers are getting sufficiently frustrated > with the current situation that they might think this is a positive > development. But meanwhile, back in the real world, we do need to > deal with the issue. You're not reading what I wrote. There are plenty of places where we read in an on-disk structure into a malloc'ed buffer, where all of the fields of that structure which have 4-byte alignment for 32-bit fields, and 2-byte alignment for 16-bit fields. We dereference those fields directly, without doing a manual copy operation. Strictly speaking, this may not be portable, since there may be architectures that do something insane with greater than 8 byte alignment requirements for 4-byte types. We (both e2fsprogs and the Linux kernel) also assume that the bit pattern field for NULL is 0x00. Strictly speaking the C standard does not require this; it only requires that if you cast an integer value of 0, the result must be the NULL pointer. For example, on the Honeywell GE-645, a NULL pointer was (segment -2, offset 0). Architectures which have odd alignments or wierd bit patterns for NULL pointers are what I am calling "insane". And I don't care about complete generic portability to weird architectures. So please don't try to make arguments about portability as a platonic ideal that we must adhere to at all costs. If we did that, we would have to manually copy each field from the on-disk structure to an in-memory structure, and that would be insanely slow. Can you imagine doing that in the TCP stack?!? I am willing to support the ARM architecture, but we need to do this in a careful way; not via some quick and dirty sed script. But please note that other architectures, including the Alpha, have always supported doing unaligned pointer fixups (even if it is slow), because if they didn't vast amounts of userspace *would* break. Regards, - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
