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Before commita94a1d7699("fuse: Manually process requests (without libfuse)"), the O_TRUNC flag when open()-ing an export would be ignored. This is because libfuse sets FUSE_CAP_ATOMIC_O_TRUNC, so the kernel lets user space handle the O_TRUNC flag, which is ignored by the fuse code for export. After the commit, FUSE_CAP_ATOMIC_O_TRUNC is not set anymore, so the O_TRUNC flag is handled by the kernel, which executes a truncate. For blockdev-based exports, this causes a regression, because opening with O_TRUNC would previously work, but results in an ENOTSUP after commita94a1d7699. For file-based exports, the fact that truncate is executed can be considered an improvement in general. However, in combination with growable=off, this still results in a practical regression in combination with virt-fw-vars, which opens its output file with O_TRUNC and previously worked with a file-based export with growable=off. After commita94a1d7699, the file is truncated upon open and then cannot grow, meaning virt-fw-vars won't be able to write the output. To fix these regressions, while keeping the improved behavior for file-based exports with growable=on, set the FUSE_CAP_ATOMIC_O_TRUNC flag again if growable=off. Cc: qemu-stable@nongnu.org Fixes:a94a1d7699("fuse: Manually process requests (without libfuse)") Signed-off-by: Fiona Ebner <f.ebner@proxmox.com> Message-ID: <20260702132256.661429-2-f.ebner@proxmox.com> Reviewed-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>