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fix(fspy): read the whole shebang line - #788
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fspy read only the first 128 bytes of a script to find its interpreter, so an interpreter path longer than 126 bytes was cut short. The traced exec then ran the truncated path, which fails or runs a different program, while the same script runs fine without tracing. Read as much as the kernel does instead: 512 bytes on macOS and 256 on Linux. Where the kernel refuses a line that doesn't fit, return ENOEXEC like it does rather than running a truncated path.
On macOS, `#` ends the shebang line like a newline does (`IS_EOL` in XNU's `exec_shell_imgact`). Reading 512 bytes instead of 128 made a `#` past the first 128 bytes part of the interpreter or its arguments, and a line without a newline in its first 512 bytes failed with ENOEXEC even when a `#` ended it. On Linux, when the 256-byte buffer fills up without a newline, the space or tab after the interpreter path can be the buffer's last byte. The kernel looks for it there before dropping that byte, but fspy dropped the byte first and refused the script. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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fspy only swaps the arm64e `/bin/sh` for its own shell when it's run by that path. Through the test's symlink it injects the arm64 preload instead, and dyld refuses to load it where SIP is off, as on the CI runners. The script still runs on Linux; on macOS the test checks that the whole 200-byte interpreter path was read, which the 128-byte read didn't do. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Motivation
fspy reads only the first 128 bytes of a script to find its interpreter. A script whose interpreter path is longer than 126 bytes runs fine on its own, but under a cached task the path gets cut short. The exec then fails with ENOENT, or runs whatever lives at the shorter path.
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