Hermit: fix readdir() #158649
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This is also how it is done on other platforms.
This optimization was already partially in place, but not used. Other platforms already do this.
Previously, we read all entries into a huge initialized buffer up front, which does not work correctly since on Hermit 0.12, getdents64 behaves more reasonable and does no longer fail on buffers which cannot hold all entries. Additionally, for each new entry, we started searching for the current position from the start instead of just saving the position directly. The new design uses a fixed-size uninitialized buffer that is read into as necessary. Co-authored-by: Martin Kröning <martin.kroening@eonerc.rwth-aachen.de>
While Hermit does not return these yet, it will do so in the future.
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On Hermit, the
sys_getdentsimplementation was flawed, as it wasn't stateful. So the caller had to provide a buffer large enough to hold alldirentsof the directory in question. Else it returnsEINVAL. The current workaround used in Rust is to grow the buffer until it is sufficiently large.hermit-os/kernel#1738 fixes the behavior of
sys_getdents. Hermit now keeps track of the directory position, and each new call to thesys_getdentsprovides the next entries.But this results in the current implementation in Rust Std being flawed: Because Hermit now returns a proper readcount,
read_dirwill only ever iterate over the first 512 bytes of directory entries, even if not alldirent64s are read.This PR fixes this with a proper implementation of the
getdents64algorithm.Remark:
This breaks compatibility with Hermit versions 0.11 and previous. We have discussed this in the Hermit team and have come to the conclusion that this is ok. We have released two newer "major" versions since, and Hermit is still in the development phase where we don't guarantee any stability and backwards compatibility in the kernel itself.