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vfs: load each memfd image under a path of its own - #67068
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pipobscure wants to merge 2 commits into
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On Linux an addon or FFI library inside a VFS is loaded from an anonymous memfd through /proc/self/fd/N, and the memfd was closed right after a successful load. The next image then got the same fd number, hence the same path, and the dynamic loader, which knows a loaded object by the path it was opened with, returned the object already loaded under that path instead of loading the new file. So the second of any two addons or libraries loaded from a VFS resolved to the first: an addon after an FFI library failed with "Module did not self-register", an FFI library after an addon had none of its own symbols, and two addons silently became the same one. Keep a memfd open for the life of the process once it has been loaded, so no two images ever share a path; MFD_CLOEXEC keeps it out of child processes. A failed load still closes its memfd. The temp-file fallback is unaffected: each image there gets a fresh mkdtemp() directory, and Windows already keeps its images until exit. Signed-off-by: Philipp Dunkel <pip@pipobscure.com>
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All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #67068 +/- ##
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- Coverage 90.65% 90.65% -0.01%
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Files 797 797
Lines 278268 278320 +52
Branches 53517 53535 +18
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+ Hits 252261 252305 +44
- Misses 16386 16397 +11
+ Partials 9621 9618 -3
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addaleax
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Oct 30, 2026
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Do we have to leak here? Can we just close after uv_dlclose()?
There is an expectation that Node.js instances clean up file descriptors opened by them unless explicitly told otherwise.
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You might have a point. I'm reworking it with a slightly better idea that makes sure we don't leak. uv_dlclode() doesn't solve the problem, since that would just lead to reused Hold for changes comint |
Keeping a loaded memfd open until exit gave each image a path of its own, but held one file descriptor per load for the life of the process: ffi's close() did not release it, nor did a worker's exit, nor a load that failed after dlopen(), so a process opening libraries in a loop would grow without bound. The fd is only needed until dlopen() returns, as the load's mapping keeps the memory alive from then on. What has to last is the path, and that depends only on the number. So move each image's fd to a number no earlier image has had, using F_DUPFD_CLOEXEC with a process-wide counter that only goes up, and close it right after the load again. Unlike closing it after dlclose(), this also holds for an object that dlclose() leaves loaded, and for addons on the main thread, which are never unloaded. Should the numbers run out, the image goes to a temp file. Signed-off-by: Philipp Dunkel <pip@pipobscure.com>
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On Linux an addon or FFI library inside a VFS is loaded from an anonymous memfd through /proc/self/fd/N, and the memfd was closed right after a successful load. The next image then got the same fd number, hence the same path, and the dynamic loader, which knows a loaded object by the path it was opened with, returned the object already loaded under that path instead of loading the new file. So the second of any two addons or libraries loaded from a VFS resolved to the first: an addon after an FFI library failed with "Module did not self-register", an FFI library after an addon had none of its own symbols, and two addons silently became the same one.
Keep a memfd open for the life of the process once it has been loaded, so no two images ever share a path; MFD_CLOEXEC keeps it out of child processes. A failed load still closes its memfd. The temp-file fallback is unaffected: each image there gets a fresh mkdtemp() directory, and Windows already keeps its images until exit.