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passthrough: fall back to caller's flags when writeback open promotio… - #263
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jiangliu merged 1 commit intoOct 10, 2026
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…n fails With writeback cache enabled, get_writeback_open_flags() promotes O_WRONLY to O_RDWR so the kernel can issue read requests through the handle. For a file the caller may only write (e.g. mode 0o200, where the owner has no read permission) the promoted open fails with EACCES, turning a valid O_WRONLY open or CREATE into a spurious failure: without writeback the kernel would never issue reads on the handle anyway. Retry with the caller's access mode when the promoted open fails with EACCES, keeping the other writeback adjustments (O_APPEND stays cleared): the handle is then genuinely write-only and a kernel READ on it fails with the real error instead. The same fallback covers CREATE, where a mode that denies read access to the owner would otherwise fail the whole creation; if the first attempt already created the file the retry answers EEXIST and the existing-file path opens it with the caller's flags. The fallback is guarded on the access mode actually having been promoted, so an explicit O_RDWR request on a write-only file still fails with EACCES. Ported from PR cloud-hypervisor#220 ("Fix O_WRONLY open for files without read permission"). Co-authored-by: EJ Campbell <ejc3@users.noreply.github.com> Signed-off-by: Jiang Liu <gerry@linux.alibaba.com>
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October 10, 2026 02:47
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With writeback cache enabled, get_writeback_open_flags() promotes O_WRONLY to O_RDWR so the kernel can issue read requests through the handle. For a file the caller may only write (e.g. mode 0o200, where the owner has no read permission) the promoted open fails with EACCES, turning a valid O_WRONLY open or CREATE into a spurious failure: without writeback the kernel would never issue reads on the handle anyway.
Retry with the caller's access mode when the promoted open fails with EACCES, keeping the other writeback adjustments (O_APPEND stays cleared): the handle is then genuinely write-only and a kernel READ on it fails with the real error instead. The same fallback covers CREATE, where a mode that denies read access to the owner would otherwise fail the whole creation; if the first attempt already created the file the retry answers EEXIST and the existing-file path opens it with the caller's flags.
The fallback is guarded on the access mode actually having been promoted, so an explicit O_RDWR request on a write-only file still fails with EACCES.
Ported from PR #220 ("Fix O_WRONLY open for files without read permission").