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Fix integer overflow in ReadFile buffer pre-allocation #29207

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Fix integer overflow in ReadFile buffer pre-allocation #29207

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5bd3812
Fix integer overflow in ReadFile buffer pre-allocation
robobun Apr 12, 2026
d08e2ff
[autofix.ci] apply automated fixes
autofix-ci[bot] Apr 12, 2026
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4 changes: 2 additions & 2 deletions src/bun.js/webcore/blob/read_file.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -384,7 +384,7 @@ pub const ReadFile = struct {

// add an extra 16 bytes to the buffer to avoid having to resize it for trailing extra data
if (!this.could_block or (this.size > 0 and this.size != Blob.max_size))
this.buffer = std.ArrayListUnmanaged(u8).initCapacity(bun.default_allocator, this.size + 16) catch |err| {
this.buffer = std.ArrayListUnmanaged(u8).initCapacity(bun.default_allocator, this.size +| 16) catch |err| {
this.errno = err;
this.onFinish();
return;
Comment on lines +387 to 390

@coderabbitai coderabbitai Bot Apr 12, 2026

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⚠️ Potential issue | 🟠 Major

Propagate OOM to system_error on the non-Windows path.

At Line 387, the catch path sets this.errno but not this.system_error; then() rejects only when system_error exists, so allocation failure can surface as a successful empty read instead of an error.

Suggested fix 
-        if (!this.could_block or (this.size > 0 and this.size != Blob.max_size))
-            this.buffer = std.ArrayListUnmanaged(u8).initCapacity(bun.default_allocator, this.size +| 16) catch |err| {
-                this.errno = err;
-                this.onFinish();
-                return;
-            };
+        if (!this.could_block or (this.size > 0 and this.size != Blob.max_size))
+            this.buffer = std.ArrayListUnmanaged(u8).initCapacity(bun.default_allocator, this.size +| 16) catch |err| {
+                this.errno = err;
+                if (err == error.OutOfMemory) {
+                    this.system_error = bun.sys.Error.fromCode(bun.sys.E.NOMEM, .read).toSystemError();
+                }
+                this.onFinish();
+                return;
+            };
🤖 Prompt for AI Agents 
Verify each finding against the current code and only fix it if needed.

In `@src/bun.js/webcore/blob/read_file.zig` around lines 387 - 390, The allocation
catch in the ArrayListUnmanaged initCapacity path currently only sets this.errno
and calls this.onFinish(), so then() won't reject; update the catch in the
non-Windows branch to also set this.system_error (e.g., assign the allocation
error or a mapped OOM system_error to this.system_error) before calling
this.onFinish() and returning, ensuring code paths in the read handler (the
this.buffer init, this.errno, this.system_error, onFinish(), and then()
rejection behavior) correctly propagate OOM to the caller.

Comment on lines 384 to 390

@claude claude Bot Apr 12, 2026

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🔴 In ReadFile.runAsyncWithFD, the OOM catch block sets only this.errno but leaves this.system_error null; since then() dispatches errors solely via system_error, an OOM during buffer pre-allocation silently resolves the JS promise with an empty string instead of rejecting. Add this.system_error = bun.sys.Error.fromCode(bun.sys.E.NOMEM, .read).toSystemError(); alongside the existing this.errno = err; at line 388.

Extended reasoning...

What the bug is and how it manifests

In ReadFile.runAsyncWithFD (the POSIX/non-UV path), when initCapacity fails with OutOfMemory (now reliably triggered by the PR's saturating-add fix passing maxInt(u52) as the capacity), the catch block executes:

this.errno = err;
this.onFinish();
return;

It sets this.errno but leaves this.system_error = null. The then() dispatcher — the only path that converts the task result into a JS callback — checks exclusively this.system_error to decide whether to call the callback with an error:

const system_error = this.system_error;
// ...
if (system_error) |err| {
    cb(cb_ctx, ReadFileResultType{ .err = err });
    return;
}
cb(cb_ctx, .{ .result = .{ .buf = buf, .total_size = total_size, .is_temporary = true } });

Because system_error is null, then() falls through to the success path, calling cb with buf = this.buffer.items which is an empty slice — the allocation never succeeded. The JS promise therefore resolves with "" instead of rejecting.

The specific code path that triggers it

  1. JS calls Bun.file("/dev/zero").slice(0, 4503599627370490).text()
  2. resolveSizeAndLastModified sets this.size = this.max_length (≈ maxInt(u52))
  3. runAsyncWithFD reaches initCapacity(bun.default_allocator, this.size +| 16) — after the PR fix, this is maxInt(u52)
  4. initCapacity returns error.OutOfMemory
  5. Only this.errno = err is set; this.system_error stays null
  6. onFinish() → then() → success callback with empty buffer

Why existing code doesn't prevent it

The then() function never checks this.errno at all; it is solely gated on this.system_error. This is an oversight: every other OOM handler in the same file (ReadFileUV lines ~702–703, ~731–732, ~764–765, ~791–792, and the ensureUnusedCapacity path) correctly sets both this.errno and this.system_error. Only this one catch block was left incomplete.

What the impact would be

A caller reading a large Blob on POSIX (e.g., a sliced /dev/zero) will receive an empty string from .text() / .arrayBuffer() / etc. without any error indication. This is silent data loss/corruption: code that checks the result value rather than expecting a rejection will silently process an empty string as if it were the file's contents.

How to fix it

Add the missing line inside the catch block:

this.buffer = std.ArrayListUnmanaged(u8).initCapacity(bun.default_allocator, this.size +| 16) catch |err| {
    this.errno = err;
    this.system_error = bun.sys.Error.fromCode(bun.sys.E.NOMEM, .read).toSystemError(); // <-- add this
    this.onFinish();
    return;
};

Step-by-step proof

  1. Bun.file("/dev/zero").slice(0, 4503599627370490).text() is awaited in JS.
  2. ReadFile.createWithCtx allocates a ReadFile with max_length = 4503599627370490 (~maxInt(u52)).
  3. resolveSizeAndLastModified: /dev/zero is not a regular file, stat.size == 0, so this.size = this.max_length = 4503599627370490.
  4. runAsyncWithFD: this.size != Blob.max_size (they happen to be equal only at exact maxInt(u52), but after saturating add the capacity request is still enormous) → initCapacity(allocator, 4503599627370507) is called.
  5. The allocator returns error.OutOfMemory. The catch block sets this.errno = error.OutOfMemory only.
  6. onFinish() is called; eventually then() runs on the JS thread.
  7. In then(): this.store != null → skip first two branches. system_error = this.system_error = null. if (system_error) |err| is false. cb is called with .result = .{ .buf = &[]{}, .total_size = 0, .is_temporary = true }.
  8. JS promise resolves with "" — no error thrown, no rejection.

Expand Down Expand Up @@ -698,7 +698,7 @@ pub const ReadFileUV = struct {
return;
}
// add an extra 16 bytes to the buffer to avoid having to resize it for trailing extra data
this.buffer.ensureTotalCapacityPrecise(this.byte_store.allocator, @min(this.size + 16, @as(usize, std.math.maxInt(bun.windows.ULONG)))) catch {
this.buffer.ensureTotalCapacityPrecise(this.byte_store.allocator, @min(this.size +| 16, @as(usize, std.math.maxInt(bun.windows.ULONG)))) catch {
this.errno = error.OutOfMemory;
this.system_error = bun.sys.Error.fromCode(bun.sys.E.NOMEM, .read).toSystemError();
this.onFinish();
Expand Down
13 changes: 13 additions & 0 deletions test/js/bun/util/bun-file-read.test.ts
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Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
import { expect, it } from "bun:test";
import { bunEnv, bunExe, isPosix } from "harness";
import { tmpdir } from "node:os";

it("offset should work in Bun.file() #4963", async () => {
Expand All @@ -9,3 +10,15 @@ it("offset should work in Bun.file() #4963", async () => {
const contents = await slice.text();
expect(contents).toBe("ntents");
});

it.skipIf(!isPosix)("reading a file blob sliced to near Blob.max_size should not crash", async () => {
await using proc = Bun.spawn({
cmd: [bunExe(), "-e", `await Bun.file("/dev/zero").slice(0, 4503599627370490).text().then(() => {}, () => {});`],
env: bunEnv,
stdout: "pipe",
stderr: "pipe",
});
const exitCode = await proc.exited;
expect(proc.signalCode).toBeNull();
expect(exitCode).toBe(0);
});
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