Stale data (zeros) returned under specific circumstances

[exFalso]

Sorry for the mysterious title. We've been implementing a fuse-rs based FS and implemented a fuzzer for simple filesystem operations like read/write/seek. We encountered an issue where our implementation was behaving very strangely, returning zeros or sometimes stale data under certain circumstances. The smallest example I reduced it to:

1. New file X is open twice, once in read mode R and once in write mode W.
2. R is read -> empty data. This step is important as it triggers a kernel read request of size 4096, and it seems as if the kernel is caching the returned data
3. W is written with N bytes -> success.
4. R is read again -> the read bytes have the correct length N, but they are all zeros. The filesystem is not contacted by the kernel at all. It seems as if the kernel has updated the file metadata during 3, but has not invalidated the cached read.

I have tried reproducing with the regular libfuse passthrough.c implementation, but the issue did not reproduce. I did notice a larger version skew between the libfuse vs fuse-rs ABI versions.

Then I tried modifying hello.rs to add the required features (write and setattr) with as minimal changes as possible, and the issue did reproduce. The code can be found here: https://github.com/zargony/fuse-rs/compare/master...decentriq:fuse-rs:aslemmer/zeros-read-bug?expand=1, note that it branches off the latest release tag, not master.

In particular, the reproducing test: https://github.com/zargony/fuse-rs/blob/10df620bf644cb4f6b4c14868c1195ff00c0ad62/examples/hello.rs#L139-L153

When running this test, the filesystem output:

$ RUST_LOG=debug cargo run --example hello /tmp/mnt
...
INFO:fuse::session: Mounting /tmp/mnt
DEBUG:fuse::request: INIT(2)   kernel: ABI 7.34, flags 0x33fffffb, max readahead 131072
DEBUG:fuse::request: INIT(2) response: ABI 7.8, flags 0x1, max readahead 131072, max write 16777216
DEBUG:fuse::request: LOOKUP(4) parent 0x0000000000000001, name "hello.txt"
DEBUG:fuse::request: OPEN(6) ino 0x0000000000000002, flags 0x8000
DEBUG:fuse::request: OPEN(8) ino 0x0000000000000002, flags 0x8001
DEBUG:fuse::request: GETXATTR(10) ino 0x0000000000000002, name "security.capability", size 0
DEBUG:fuse::request: SETATTR(12) ino 0x0000000000000002, valid 0x208
DEBUG:fuse::request: WRITE(14) ino 0x0000000000000002, fh 0, offset 0, size 4, flags 0x6
DEBUG:fuse::request: GETATTR(16) ino 0x0000000000000002
DEBUG:fuse::request: READ(18) ino 0x0000000000000002, fh 0, offset 0, size 4096
DEBUG:fuse::request: WRITE(20) ino 0x0000000000000002, fh 0, offset 4, size 12, flags 0x6
DEBUG:fuse::request: GETATTR(22) ino 0x0000000000000002
DEBUG:fuse::request: FLUSH(24) ino 0x0000000000000002, fh 0, lock owner 31099430268040709
DEBUG:fuse::request: RELEASE(26) ino 0x0000000000000002, fh 0, flags 0x8001, release flags 0x0, lock owner 0
DEBUG:fuse::request: RELEASE(28) ino 0x0000000000000002, fh 0, flags 0x8000, release flags 0x0, lock owner 0

The test output:

$ cargo test --example hello stale_data_bug -- --nocapture
...
running 1 test
buffer1 [73, 110, 105, 116]
buffer2 [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
test stale_data_bug ... ok

(I've added an initial write of "Init" to demonstrate that the basic write functionality works)

As you can see, the first READ request comes through fine, however after the write of "Hello World!", no new read requests arrive, the kernel simply returns zeros.

Any ideas? Is this a kernel bug? Or perhaps an ABI skew issue? I'm running a 5.15.63 kernel

[exFalso]

Turns out the issue is with open() returning the same flags that were passed to it, which inadvertently set the FOPEN_KEEP_CACHE flag, causing the kernel cache to not be invalidated properly.