From: "ivoanjo (Ivo Anjo) via ruby-core" Date: 2024-06-20T09:53:47+00:00 Subject: [ruby-core:118364] [Ruby master Bug#20586] Some filesystem calls in dir.c are missing error handling and can return incorrect results if interrupted Issue #20586 has been updated by ivoanjo (Ivo Anjo). Eregon (Benoit Daloze) wrote in #note-4: > Is the SIGPROF handler installed with [sigaction()](https://man7.org/linux/man-pages/man2/sigaction.2.html) without the `SA_RESTART` flag? > Does it still happen if the `SA_RESTART` is passed to `sigaction()`? If yes it sounds like a possible bug of gcsfuse. Unfortunately the semantics aren't as simple as that :/ Both the reproducer I shared above as well [`readdir-bug-repro.c`](https://bugs.ruby-lang.org/attachments/9751) as well as [the Datadog Ruby profiler](https://github.com/DataDog/dd-trace-rb/blob/4b1901101c0f4d1033558bd9c0ff2e1c2b963bc7/ext/datadog_profiling_native_extension/setup_signal_handler.c#L35) set `SA_RESTART` when adding the signal handler. The [man page](https://man7.org/linux/man-pages/man7/signal.7.html#:~:text=Interruption%20of%20system%20calls%20and%20library%20functions%20by%20signal%20handlers) has a section mentioning some system calls are not restarted, regardless of the flag. > The following interfaces are never restarted after being > interrupted by a signal handler, regardless of the use of > SA_RESTART; they always fail with the error EINTR when > interrupted by a signal handler: > (...) `readdir` is not mentioned in this section, but it's also not mentioned in the section about calls that are restarted. So it's in a weird limbo. mame (Yusuke Endoh) wrote in #note-5: > It is probably that the FUSE driver throws EINTR when communicating, but it is not clear what kind of processing is done before and after the communication. It is possible that the internal state is inconsistent after readdir returns EINTR. > > Then, should we rewinddir and try again? Or do we need to closedir and opendir again? > > I don't think this is a problem we should deal with in our imagination. If we are serious about this problem, we should start by reporting the problem to the Linux kernel and asking them to clarify the behavior on their manpage. Yeah -- I suspect the kernel/FUSE should hide a lot of these, otherwise most userland apps will get it wrong, but on the docs I could find it's not clear this behavior is entirely expected. > As for Ruby, I think it is good to raise an exception by `rb_sys_fail` for the time being. This is my thinking as well -- if this does turn out to be a linux/FUSE/etc bug, it'll probably take a long time to roll out the fix so having Ruby do the right thing in the face of the weird issue is probably worth it? ---------------------------------------- Bug #20586: Some filesystem calls in dir.c are missing error handling and can return incorrect results if interrupted https://bugs.ruby-lang.org/issues/20586#change-108870 * Author: ivoanjo (Ivo Anjo) * Status: Open * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN ---------------------------------------- #### Background Hey! I work for Datadog on the Ruby profiler part of the [`datadog` (previously `ddtrace`)](https://github.com/datadog/dd-trace-rb) gem. A customer reached [out with an issue](https://github.com/DataDog/dd-trace-rb/issues/3450) where enabling the profiler made `Dir.glob` return no files for a given folder: Without profiler: ``` irb(main):002:0> Dir.glob('/gcsfuse/t*') => ["/gcsfuse/test.html", "/gcsfuse/test.txt"] ``` With profiler: ``` irb(main):002:0> Dir.glob('/gcsfuse/t*') => [] ``` It turns out the issue is related to missing error handling in `dir.c`. The Datadog Ruby profiler, like stackprof, pf2 and vernier, uses unix signals to interrupt the currently-active thread and take a sample (usually `SIGPROF`). When some system calls get interrupted by a signal, they return an [EINTR error code](https://man7.org/linux/man-pages/man7/signal.7.html#:~:text=Interruption%20of%20system%20calls%20and%20library%20functions%20by%20signal%20handlers) back to the caller. Consider for instance the implementation of `dir_each_entry` in `dir.c`: ```c static VALUE dir_each_entry(VALUE dir, VALUE (*each)(VALUE, VALUE), VALUE arg, int children_only) { struct dir_data *dirp; struct dirent *dp; IF_NORMALIZE_UTF8PATH(int norm_p); GetDIR(dir, dirp); rewinddir(dirp->dir); IF_NORMALIZE_UTF8PATH(norm_p = need_normalization(dirp->dir, RSTRING_PTR(dirp->path))); while ((dp = READDIR(dirp->dir, dirp->enc)) != NULL) { // ... do things } return dir; } ``` If `READDIR` returns `NULL`, then `dir_each_entry` assumes it has iterated the entire directory. But looking [at the man page for `readdir`](https://man7.org/linux/man-pages/man3/readdir.3.html) we see the following sharp edge (emphasis mine): > It returns NULL on reaching the end of the directory stream **or if an error occurred**. So what's happening in this situation is: `readdir` gets interrupted, returns `NULL` + sets errno to `EINTR`. But `dir_each_entry` doesn't check `errno`, so rather than raising an exception to flag the issue, it treats it as if the end of the directory has been reached. #### How to reproduce Reproducing this is somewhat annoying, because it's dependent on timing: the signal must arrive at the exact time the dir API is getting executed. I was able to reproduce this every time by using the [google cloud `gcsfuse`](https://cloud.google.com/storage/docs/gcs-fuse) tool. This somewhat makes sense -- a remote filesystem is much slower than a local one, so there's a much bigger window of opportunity for a signal to arrive while the system call is blocked. Here's an example I included in https://github.com/DataDog/dd-trace-rb/pull/3720: ``` # Not shown: Set up a trial google cloud account, install gcsfuse, create a cloud storage bucket and put in some test files $ gcsfuse test_fs_dd_trace_rb fuse-testing/ $ ls fuse-testing/ hello.txt test.html test.txt # Not shown: Add `datadog` gem to `Gemfile` $ DD_PROFILING_ENABLED=true DD_PROFILING_DIR_INTERRUPTION_WORKAROUND_ENABLED=false bundle exec ddprofrb exec ruby -e "Datadog::Profiling.wait_until_running; pp Dir.children('fuse-testing/')" [] ``` Let me know if you'd like me to try to create a reproducer that does not depend on the `datadog` gem. #### Additional notes I've spent quite some time looking at the `dir.c` sources, and here's the full list of APIs that suffer from issues: * `dir_each_entry` does not check `errno`; all of its users have interruption bugs * `dir_tell` will return -1 instead of the correct position (which means that passing -1 to `dir_seek`/`dir_set_pos` will cause it to not list the directory properly) * `do_opendir` an error in system calls will only sometimes be turned into a raised exception * Indirect callers that pass in rb_glob_error as errfunc: rb_glob, Dir.[], Dir.glob * Indirect callers that pass in 0 as errfunc: ruby_glob, ruby_brace_glob * `glob_opendir` does not check errno; all of its users have interruption bugs * `glob_getent` does not check errno; all of its users have interruption bugs * `nogvl_dir_empty_p` does not check errno (of readdir! it actually checks for opendir); all of its users have interruption bugs Less sure about these: * `do_stat`/`do_lstat` will turn errors into warnings (unclear if enabled or disabled by default) * `need_normalization` calls `fgetattrlist` / `getattrlist` and all errors `(ret != 0)` are treated in the same way * `rb_glob_error` is and `rb_glob_caller` leave exceptions as pending and rely on callers to raise them properly * Error handling of `rb_home_dir_of` and `rb_default_home_dir` are a bit suspicious As a workaround in the Datadog Ruby profiler, in https://github.com/DataDog/dd-trace-rb/pull/3720 I've added monkey patches to all of the Ruby-level APIs that use the above functions and mask out `SIGPROF` so these calls are never interrupted. This solution is does successfully work around the issue, although it prevents the profiler from sampling during these system calls, which will mean less visibility if e.g. these calls are taking a long time. And well, maintaining monkey patches is always problematic for future Ruby compatibility. ---Files-------------------------------- readdir-bug-repro.c (2.11 KB) -- https://bugs.ruby-lang.org/ ______________________________________________ ruby-core mailing list -- ruby-core@ml.ruby-lang.org To unsubscribe send an email to ruby-core-leave@ml.ruby-lang.org ruby-core info -- https://ml.ruby-lang.org/mailman3/postorius/lists/ruby-core.ml.ruby-lang.org/