Just to expand a little on Paul's response On Tue, 16 Sept 2025 at 10:14, Paul Eggert <egg...@cs.ucla.edu> wrote:
> > 2. The other odd thing in the trace is that there are some kind of odd > > shenanigans going on with the two identical binary files, and a > > third temporary file that cp makes and then removes. > The point is that they're not *supposed* to be "identical binaries"; they're supposed to be multiple names for ONE file, because you specified --preserve=all, which includes replicating the original pattern of (hard) links in the target. > > I don't really > > follow the logic of the series of "linkat"s, "renameat"s, and > > "unlinkat"s around lines 70-80 of the trace, but while I could > > accept that they're somehow useful, The "link" and "linkat" syscalls won't overwrite an existing target, as this line shows in the strace output: linkat(3, "./build-script-build", 3, "./build_script_build-ed34004021de38b5", 0) = -1 EEXIST (File exists) To work around this, it follows up this sequence: linkat(3, "./build-script-build", 3, "./CuYCWUCU", 0) = 0 renameat(3, "./CuYCWUCU", 3, "./build_script_build-ed34004021de38b5") = 0 This works because the "rename" & "renameat" syscalls *will* overwrite a target, and moreover, they guarantee atomicity while doing so (though that's arguably not relevant here). (You might like to submit a Linux kernel feature request to implement an extra option flag for 'linkat' to do the same, perhaps AT_REPLACE.) I have to admit I'm a bit puzzled by unlinkat(3, "./CuYCWUCU", 0) = 0 which would appear to be successfully removing a filename that no longer exists. > the trace doesn't show any equivalent writes to the target > > directory, only the ones in the source directory Note that all the "*at" syscalls above reference filedescriptor 3, which was obtained earlier: openat(AT_FDCWD, "/home/kye/test", O_RDONLY|O_PATH|O_DIRECTORY) = 3 meaning, they're all relative to /home/kye/test. This contrasts with the ones that use 'AT_FDCWD' which treats relative paths as relative to the calling process's cwd. -Martin
