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A MAP_PRIVATE mapping never writes back to the file, and the kernel accepts PROT_READ | PROT_WRITE on an O_RDONLY fd for it. mmap_file now opens read-only when the flags select MAP_PRIVATE, so a 0444 file, a file on a read-only mount, or a running executable can be mapped with shared: false. MAP_SHARED keeps O_RDWR. The read-only open succeeds on a directory where O_RDWR failed with EISDIR, so mmap_file checks st_mode after fstat and keeps that error. O_NONBLOCK keeps the open of a FIFO from blocking for a writer.
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No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info
WalkthroughPrivate mappings now open files read-only, while shared mappings retain read-write access. Directory mappings return ChangesRead-only private mmap support
Suggested reviewers: Priority: ➖ Normal Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to
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| Check name | Status | Explanation |
|---|---|---|
| Title check |  |
The title clearly and concisely identifies the primary change: opening files read-only for private mappings. |
| Description check | |
The description explains the problem, implementation, behavior, trade-offs, linked issue, and verification results. It does not use the exact template headings, but it contains the required informatio… |
| Linked Issues check | |
Issue #46617 requires readable files to map without write access, at least for shared: false. src/sys/lib.rs selects O_RDONLY | O_NONBLOCK for MAP_PRIVATE and keeps O_RDWR for shared mapping… |
| Out of Scope Changes check | |
The source change implements issue #46617. The directory check preserves the previous error after changing the open mode. O_NONBLOCK preserves non-blocking FIFO behavior. The tests verify the new pr… |
Full details: Description check
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Explanation
The description explains the problem, implementation, behavior, trade-offs, linked issue, and verification results. It does not use the exact template headings, but it contains the required information and is sufficiently complete.
Full details: Linked Issues check
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Explanation
Issue
#46617requires readable files to map without write access, at least forshared: false.src/sys/lib.rsselectsO_RDONLY | O_NONBLOCKforMAP_PRIVATEand keepsO_RDWRfor shared mappings. Thefstatmode check preservesEISDIRafter the read-only open. Tests cover a running executable, a mode-0444 file, directories, and FIFOs. The documentation describes the newsharedbehavior. Shared mappings still require write access because writes must reach the file; this is consistent with the issue'sat least shared: falserequirement and the writableUint8ArrayAPI.
Full details: Out of Scope Changes check
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Explanation
The source change implements issue
#46617. The directory check preserves the previous error after changing the open mode.O_NONBLOCKpreserves non-blocking FIFO behavior. The tests verify the new private-mapping behavior and preserved errors. The type documentation explains the changed file-access behavior. No unrelated changes are shown in the reviewed diff.
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Actionable comments posted: 2
- 🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @packages/bun-types/bun.d.ts:
- Around line 5375-5377: Qualify the `shared: false` copy-on-write guarantee in
the file-mapping option documentation and the `mmap` description: on Linux with
`sync: true`, the mapping is shared and writes can reach the file. Keep the
existing private-write description for other configurations.
Review comments at @test/js/bun/util/mmap.test.js:
- Line 151: Replace the dynamic require("fs") call in the child script with a
module-scope import of readFileSync from node:fs, and use that imported function
to read file. Keep the child script focused on testing Bun.mmap.
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packages/bun-types/bun.d.tssrc/sys/lib.rstest/js/bun/util/mmap.test.js
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Updated 4:30 PM PT - Oct 23rd, 2026
✅ @robobun, your commit a82ce140e61f0ff376a2315c9b5337afacda479e passed in 🧪 To try this PR locally: bunx bun-pr 46637That installs a local version of the PR into your bun-46637 --bun |
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline findings, I also checked the sync: true path: src/runtime/api/BunObject.rs:1426 replaces the flags with MAP_SHARED_VALIDATE | MAP_SYNC, which masks to MAP_SHARED | MAP_PRIVATE, so private is false and the file is still opened O_RDWR, matching the new JSDoc. The new EISDIR early return sits after CloseOnDrop::new(fd), so the fd is released on that path.
Extended reasoning...
The change makes bun_sys::mmap_file open the file read-only for MAP_PRIVATE mappings, adds an EISDIR check after fstat, updates the Bun.mmap JSDoc, and adds four tests; no auth, crypto or injection surface is touched. Inline findings were reported (the O_NONBLOCK lease-break behavior and a test that does not fail on the base branch on newer kernels), so this is a findings-present ruled-out note rather than an approval.
| // O_NONBLOCK: a read-only open of a FIFO blocks until a writer opens | ||
| // it, while O_RDWR does not. Keep the open immediate in both modes. | ||
| let open_flags = if private { | ||
| O::RDONLY | O::NONBLOCK |
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🟡 (optional) Linux callers mapping a file that another process holds a write lease on now get an immediate EAGAIN from open instead of a mapping. src/sys/lib.rs:3458 adds O::NONBLOCK to the private-mapping open; the kernel's lease break (fcntl F_SETLEASE, used by knfsd write delegations and Samba kernel oplocks) returns EWOULDBLOCK to an O_NONBLOCK opener instead of waiting for the holder to release. The base O_RDWR open waited out the break and then succeeded. Fix: keep the FIFO open non-blocking but let regular files wait out a lease break, e.g. on EAGAIN from the O_NONBLOCK open retry once without O_NONBLOCK (a read-only FIFO open never returns EAGAIN), or add O_NONBLOCK only when a prior stat says the path is not a regular file. [also at: src/sys/lib.rs:3461 - On a Linux host where another process holds a write lease on the file (nfsd delegations, Samba kernel oplocks, any F_SETLEASE user), Bun.mmap(path, { shared: false }) now throws EAGAIN from open instead of waiting for the lease break and succeeding as the base does.]
Why this was flagged
Trigger: Bun.mmap(path, { shared: false }) on Linux where another process holds a write lease on path via fcntl(F_SETLEASE, F_WRLCK) — the kernel NFS server grants these as write delegations and Samba does with kernel oplocks. The call reaches mmap_file through src/runtime/api/BunObject.rs:1444 with flags == MAP_PRIVATE. src/sys/lib.rs:3454-3462 then opens with O::RDONLY | O::NONBLOCK. In the kernel, __break_lease (fs/locks.c) notifies the holder and, because mode & O_NONBLOCK is set, returns -EWOULDBLOCK immediately instead of waiting up to lease-break-time. EWOULDBLOCK is EAGAIN on Linux, so the user gets EAGAIN ... open '<path>' thrown from Bun.mmap. On the base branch the open was O::RDWR without O_NONBLOCK, so the open blocked until the lease was released or timed out and the mapping succeeded. The comment at src/sys/lib.rs:3455 justifies O_NONBLOCK only for FIFOs; it does not consider that O_NONBLOCK also changes open() on regular files under a lease.
Verification: src/sys/lib.rs:3454-3461 opens with O::RDONLY | O::NONBLOCK. The Linux openat wrapper (lib.rs:1886-1889) maps the errno straight to Error::from_code_int(e, Tag::open).with_path(..) with no EAGAIN retry, and BunObject.rs:1446-1448 throws it to JS. On the base, open(path, O::RDWR, 0) had no O_NONBLOCK, so it blocked through the lease break and then succeeded.
| // A running executable cannot be opened for writing (ETXTBSY on Linux), | ||
| // even by root. A private mapping only needs a readable file. | ||
| it("shared: false maps a running executable", () => { | ||
| const map = Bun.mmap(process.execPath, { shared: false, size: 4096 }); |
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🟡 nit (optional): on Linux 6.11 and newer this test passes on the base branch too, so it does not prove the read-only open. The comment at test/js/bun/util/mmap.test.js:125 says a running executable cannot be opened for writing, but the kernel dropped that ETXTBSY check in 6.11, so the base O_RDWR open of a writable bun binary succeeds and the mapping compares equal. Fix: make the test depend on the read-only open regardless of kernel, for example map a chmod 0444 copy or a file on a read-only location from the unprivileged child instead of process.execPath, or state in the comment that only the 0444 test discriminates.
Why this was flagged
The test at test/js/bun/util/mmap.test.js:127-138 maps process.execPath with shared: false and only checks the bytes match a readSync. On the base branch mmap_file opened O_RDWR (src/sys/lib.rs:3446 before the diff); on kernels before 6.11 that failed with ETXTBSY for the running binary, which is what the comment at line 125-126 and the PR text rely on. Linux 6.11 removed deny_write_access from exec (commit "fs: don't block i_writecount during exec"), so on 6.11+ (this host runs 6.18) O_RDWR on a writable bun binary succeeds and the base passes the test. On macOS there is no ETXTBSY for running executables either, so the test only discriminates when the binary itself is not writable by the test user. The 0444 child test at line 141 still fails on base, so the PR is covered, but this test's comment states a kernel fact that no longer holds and the test is vacuous on current kernels.
Verification: nit. On Linux 6.11 or newer with a bun binary the test user can write, the base open(path, O::RDWR, 0) (src/sys/lib.rs:3446 before the diff) succeeds because the kernel dropped the ETXTBSY refusal, so the test at test/js/bun/util/mmap.test.js:127-138 passes on base too and does not exercise the read-only open. The sibling 0444 test (mmap.test.js:141-186) still proves the fix.
Fixes #46617
Problem
Bun.mmap(path, { shared: false })throwsEACCES: permission denied, open '<path>'on a file the process can read but not write (mode 0444),EROFSon a read-only mount, andETXTBSYon a running executable.bun_sys::mmap_file(src/sys/lib.rs:3446) always opens the file withO_RDWR. AMAP_PRIVATEmapping never writes to the file, so the write access is not needed.O_RDWRopen also copies the whole file from the image layer into the container layer before it is mapped.Fix
mmap_filepicks the open mode from theMAP_*flags it already receives.MAP_PRIVATEopensO_RDONLY | O_NONBLOCK.MAP_SHARED(andMAP_SHARED_VALIDATE | MAP_SYNC) keepsO_RDWR. The mapping keepsPROT_READ | PROT_WRITE: the kernel allows that on a read-only fd for a private mapping.O_RDWRfailed, sommap_filechecksst_modeafter the existingfstatand keeps theEISDIRerror fromopen.O_NONBLOCKkeeps the open of a FIFO from blocking for a writer, asO_RDWRdid not block either.Bun.mmap(path)and{ shared: true }on a non-writable file still throwEACCES/EROFS. A shared mapping writes through to the file, and the docs promise that. A read-only shared map would make every write aSIGSEGV.test/js/bun/util/mmap.test.js(four new tests, two fail on main: theprocess.execPathone fails withETXTBSYas root). Alsotest/integration/bun-types/bun-types.test.tsandcargo check -p bun_sysfor linux, macOS, windows and musl targets.Background
Bun.mmapmaps a file into memory and returns aUint8Arrayover the mapping.shared: falseselectsMAP_PRIVATE: writes stay in this process (copy-on-write) and never reach the file.shared: true(the default) selectsMAP_SHARED: writes go to the file.mmap(2)requires a writable fd only forMAP_SHAREDwithPROT_WRITE. ForMAP_PRIVATEany readable fd is enough, whatever thePROT_*bits are.src/runtime/api/BunObject.rsby passing open flags and prot alongside the map flags. It produces the same syscalls, widens a cross-crate signature for one caller, and needs acfg(linux)arm forMAP_SYNC. The helper already has the map flags and is the only place that opens the file, so the choice lives there.Downsides
Bun.mmap(path)with the defaultshared: trueon a read-only file still throws. Users who want to map such a file must passshared: false. The JSDoc onMMapOptions.sharednow says so.mmap_filegrows from 701 to 841 bytes (nm -S, release build ofbun_sys, main vs PR). Per call: one flag mask and onest_modecompare more. Syscalls per call stay at four (open,fstat,mmap,close), read from the code (strace is not available in this container).Bun.mmap(lowerFile, { shared: false }): yes on main, no with this PR (measured with a lower/upper/work mount).{}still copies up in both.Notes
EACCESthree times on main. With this PR it prints4for{ shared: false }andEACCESfor{}and{ shared: true }.MAP_PRIVATE+PROT_READ | PROT_WRITEon anO_RDONLYfd succeeds and the write stays private.MAP_SHARED+PROT_WRITEon anO_RDONLYfd fails withEACCES.MAP_SHARED_VALIDATEis0x3on Linux, that isMAP_SHARED | MAP_PRIVATE, so the classification masks both bits and compares againstMAP_PRIVATE.Bun.mmap(fifo, { shared: false })and{ shared: true }both fail at once withEINVALfrommmap(st_size is 0). WithoutO_NONBLOCKthe read-only open would block until a writer opens the FIFO.unprivilegedSpawnOptionsfrom the harness, so it runs as uid 65534 when the suite runs as root.strace,perf,bloaty. The syscall count comes from the code path, and the size delta fromnm -Son thebun_sysrelease rlib.no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/util/mmap.test.js