@samitouri / QOSamiQemu / commits / b9ad1c4396

block/export/fuse: fix regression with O_TRUNC when export is not growable

Before commit a94a1d7699 ("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 commit a94a1d7699. 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 commit a94a1d7699, 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>

Fiona Ebner committed Jul 2, 2026 at 15:22 UTC b9ad1c4396e7eb376f0f8dc7d20538980cfa08f2

1 file changed +12

block/export/fuse.c

+12

@@ -859,6 +859,18 @@ fuse_co_init(FuseExport *exp, struct fuse_init_out *out,

859 uint32_t supported_flags = FUSE_ASYNC_READ | FUSE_ASYNC_DIO;

860 uint32_t flags2 = 0;

861

862 + if (!exp->growable) {

863 + /*

864 + * Back when libfuse was used, it would always set this flag and thus

865 + * the kernel did not execute a truncate itself and passed along O_TRUNC

866 + * to user space. Continue setting the flag for backwards compatibility

867 + * when the export is not growable to avoid issues with O_TRUNC, i.e.

868 + * blockdev-based exports running into ENOTSUP and file-based exports

869 + * with growable=off to be truncated and then stuck with size 0.

870 + */

871 + supported_flags = FUSE_ATOMIC_O_TRUNC;

872 + }

873 +

874 if (in->major != 7) {

875 error_report("FUSE major version mismatch: We have 7, but kernel has %"

876 PRIu32, in->major);