On Fri, Nov 21, 2014 at 1:18 PM, Eric Blake <eblake@xxxxxxxxxx> wrote: > On 11/21/2014 01:48 PM, Pádraig Brady wrote: >> On 21/11/14 19:55, Andy Lutomirski wrote: >>> On Fri, Nov 21, 2014 at 10:39 AM, Pádraig Brady <P@xxxxxxxxxxxxxx> wrote: >>>> On 21/11/14 18:09, Andy Lutomirski wrote: >>>>> On Fri, Nov 21, 2014 at 6:17 AM, Pádraig Brady <P@xxxxxxxxxxxxxx> wrote: >>>>>> If 'a' and 'b' are hardlinks, then the command `mv a b & mv b a` >>>>>> can result in both being removed as mv needs to emulate the >>>>>> move with an unlink of the source file. This can only be done >>>>>> without races in the kernel and so mv was recently changed >>>>>> to not allow this operation at all. mv could safely reintroduce >>>>>> this feature by leveraging a new flag for renameat() for which >>>>>> an illustrative/untested patch is attached. >>>>> >>>>> ISTM the issue is that rename(2) does nothing if the source and dest >>>>> are hardlinks to each other. Is that intentional? I don't see that >>>>> behavior as required in the POSIX rename docs. > > Yes, POSIX unfortunately requires that behavior: > > http://pubs.opengroup.org/onlinepubs/9699919799/functions/rename.html > > "If the old argument and the new argument resolve to either the same > existing directory entry or different directory entries for the same > existing file, rename() shall return successfully and perform no other > action." > >>>> >>>> It's surprising and annoying that existing systems do this, >>>> but they're conforming to the wording of POSIX I thinkg as >>>> it says that rename() does nothing when the source and dest >>>> _file_ is the same. What POSIX really meant I suppose was _file name_. >>>> Eric, perhaps an adjustment could be proposed to POSIX, as I can't >>>> see anything relying on the current behavior? > > The POSIX wording describes existing practice of all implementations, > and as annoying as it is, requiring all implementations to change would > be too invasive, and going against that wording without the use of an > extension to renameat would catch software off-guard that is expecting > the baked-in POSIX semantics. > > That said, would you still like me to take a stab at a proposal to the > POSIX folks that would relax the requirements to allow > implementation-defined behavior when the two arguments to rename > describe the same file but via different directory entries? Note that I > am NOT proposing a change to rename("a", "a") which must always succeed > (true even when two spellings are different but still resolve to the > same directory entry, as in rename("a", "./a")). Rather, this is only a > question about the behavior of rename("a", "b") when they are two > different hardlinks resolving to the same file. > > At any rate, regardless of what POSIX says, I'm totally in favor of a > renameat flag that gives us fine-tune control over the behavior; we can > implement it now as an extension, and once it hits mainline kernel, I > would have no problems proposing that flag for POSIX standardization > (the POSIX folks have a thing for preferring existing implementations, > after all) renameat has no flags -- this would be renameat2. Standardizing that might be quite nice (at least the RENAME_NOREPLACE part). My two cents: the new flag could be RENAME_HARDLINKS. --Andy > > -- > Eric Blake eblake redhat com +1-919-301-3266 > Libvirt virtualization library http://libvirt.org > -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
