[PULL 13/17] block/export/fuse: fix regression with O_TRUNC when export is not growable

From: Fiona Ebner <[email protected]>

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: [email protected]
Fixes: a94a1d7699 ("fuse: Manually process requests (without libfuse)")
Signed-off-by: Fiona Ebner <[email protected]>
Message-ID: <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>
---
 block/export/fuse.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/block/export/fuse.c b/block/export/fuse.c
index c0e8dfb6430..9ae22a2e003 100644
--- a/block/export/fuse.c
+++ b/block/export/fuse.c
@@ -859,6 +859,18 @@ fuse_co_init(FuseExport *exp, struct fuse_init_out *out,
     uint32_t supported_flags = FUSE_ASYNC_READ | FUSE_ASYNC_DIO;
     uint32_t flags2 = 0;
 
+    if (!exp->growable) {
+        /*
+         * Back when libfuse was used, it would always set this flag and thus
+         * the kernel did not execute a truncate itself and passed along O_TRUNC
+         * to user space. Continue setting the flag for backwards compatibility
+         * when the export is not growable to avoid issues with O_TRUNC, i.e.
+         * blockdev-based exports running into ENOTSUP and file-based exports
+         * with growable=off to be truncated and then stuck with size 0.
+         */
+        supported_flags = FUSE_ATOMIC_O_TRUNC;
+    }
+
     if (in->major != 7) {
         error_report("FUSE major version mismatch: We have 7, but kernel has %"
                      PRIu32, in->major);
-- 
2.55.0