mknodat is not implemented on darwin-xnu
| Originator: | wwcohen | ||
| Number: | rdar://FB9862426 | Date Originated: | 01/28/2022 |
| Status: | Open | Resolved: | |
| Product: | Product Version: | ||
| Classification: | Reproducible: |
mknodat (https://pubs.opengroup.org/onlinepubs/9699919799/functions/mknodat.html) has not been implemented on darwin (only mknod is present: https://github.com/apple/darwin-xnu/blob/8f02f2a044b9bb1ad951987ef5bab20ec9486310/bsd/sys/stat.h#L567). In situations where we need to avoid race conditions with later setting permissions, we must avoid using mknod on the full path instead. That said, at the moment, the only safe way to implement mknodat on darwin is to use the undocumented syscall pthread_fchdir (349) (https://github.com/apple/darwin-xnu/blob/a1babec6b135d1f35b2590a1990af3c5c5393479/bsd/kern/syscalls.master#L529) to set the cwd for the current thread only. This was first noticed in 2018 (https://lists.nongnu.org/archive/html/qemu-devel/2018-05/msg07340.html) with attempts to bring 9pfs functionality to QEMU macOS to mirror its capabilities on Linux, and has been brought to my attention as I have tried to bring this proposed functionality in QEMU forward a few years later (https://lists.nongnu.org/archive/html/qemu-devel/2021-11/msg04331.html). While having mknodat implemented is the ultimate end goal, in addition please do not remove this syscall until that happens.
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