On Wed, Jul 11, 2012 at 10:04:51AM -0500, Eric Sandeen wrote: > On 7/11/12 8:20 AM, Gordan Bobic wrote: > > I would like to bring the following bug report to people's > > attention: > > > > https://bugzilla.redhat.com/show_bug.cgi?id=680090 > > > > The issue is that the code used in e2fsprogs WRT allocating unaligned > > buffers and then casting them into structs is non-portable and > > dangerous - eyewateringly dangerous in something like e2fsprogs where > > it can lead to thorough trashing of the file system. I think you're being a bit over-dramatic here; can you point out a place where this is actually going to lead to "trashing of the file system"? 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. There's an awful lot of userspace code out there which assumes this works. Doing a quick grep, I'm seeing code in the blkid library (which has now been moved into util-linux) which does this. I'm also seeing one case of it in the tdb library (which we imported from Samba). That being said, doing fixups is slow since it requires a kernel fault, and I have no problem with trying to avoid this practice. > > Can something be done to improve the portability of the userspace > > code? Yes, someone can send me patches. :-) Ranting and raving about how file systems are being trashed will be met with the observation that no one has actually reported this. And this won't be the only place where the Linux kernel code makes assumptions which are not guaranteed by the C standard, but the official position that Linus has taken is any architecture (or C compiler) is insane, and it's up to the architecture to provide workarounds. > Many of these are the result of things like: > > char buf[4096] = ""; > struct fiemap *fiemap = (struct fiemap *)buf; > > > Ted, is this construct just an attempt to avoid malloc/free for > simplicity of the code? Probably; it's the way Kalpak Shaw wrote the code when he submitted it. And while I do have higher standards of portability that the Linux kernel, this is not one of the things that I test for. Please do note that malloc() is guaranteed to return a block of memory which satisifies the worse case alignment requirements of the architecture, so there are plenty of places where we cast a buffer pointer to an int * or a struct * which are completely safe. It's only when someone is allocating space on the stack where this is potentially problematic. > 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. > (IIRC "make gcc-wall" will also emit warnings for casts which change > natural alignment, among other things) I'd have to check to be sure, but I don't think so, since it would have way too many false positives. We *do* have code where we take char *'s and and then cast them to some other pointer type, and then dereference them. And we do currently assume that it is safe to do this for on-disk data structures which are 4 byte aligned, in the directory entry code, for example. I will *not* accept a patch which uses memcpy to copy each field in the on-disk superblock, or directory entry, into an int, just in case there is some insane architecture which requires that 4 byte integers be 32-byte aligned, or something else insane like that. 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. But that being said, changing places where we allocate a char [] buffer on the stack and assuming that it is 4-byte aligned does seem reasonable. But I see this as an optimization issue more than I do a performance issue.... 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
