[RFC] Determine Blob map_access permissions via fcntl instead of relying on defaults#42
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gurchetansingh merged 2 commits intomagma-gpu:mainfrom Mar 3, 2026
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[XXX: This will be sent to Mesa] Signed-off-by: Val Packett <val@invisiblethingslab.com>
Instead of relying on defaults, use the newly available detection function for the map access mode that takes into account the fd mode and the seals. Signed-off-by: Val Packett <val@invisiblethingslab.com>
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Hi @valpackett I've been testing muvm with X11 clients like xeyes which doesn't really exercise PipeWire SHM streaming directly. |
gurchetansingh
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Feb 23, 2026
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Looks terrific, looks ready to merge when out of RFC mode.
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Finally runtime-tested, and now submitted the Mesa part to Mesa: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40132 |
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In libkrun, we've had quite a bit of trouble with these permissions, since with the introduction of PipeWire support the mode for shm/memfd was changed to
RW, but that ended up crashing Wayland clients when the compositor would send us sealed-but-RDWR FDs (e.g. Smithay-based niri), so I've debugged that and added a check for the seals, and later discovered that wlroots-based compositors send RDONLY-but-not-write-sealed FDs and we'd still map these asRWand explode…I'm not 100% sure if
RWin this kind of mapping is necessary for PipeWire SHM streaming, interestingly #36 here does not change this (cc @dorindabassey — does PW just work withREADmappings for you?)… but once we start doing more interesting things like exporting dmabufs via PW things should definitely get more, well, interesting (I don't expect screen capture dmabufs to be writable heh). In any case relying on hardcoded defaults seems inherently limited.As the bulk of this is Mesa code, I'd like to just get initial review here, if this all seems good and correct I'll prepare a merge request for Mesa.
Also: only compile tested for now.