On 18 Nov 2020, at 21:52, Stefan Esser <s...@freebsd.org> wrote: > Am 18.11.20 um 22:15 schrieb Jessica Clarke: >> On 18 Nov 2020, at 19:44, Stefan Eßer <s...@freebsd.org> wrote: >>> + /* >>> + * Check for some other thread already having >>> + * set localbase - this should use atomic ops. >>> + * The amount of memory allocated above may leak, >>> + * if a parallel update in another thread is not >>> + * detected and the non-NULL pointer is overwritten. >>> + */ >> Why was this committed with a known racy/leaky implementation? > > Because the alternatives that I offered for discussion were > less acceptable.
That has no bearing over whether this one is. >> What happens if I set the value with a sysctl and call this? > > You'll get the value set with sysctl, unless overridden by the > environment variable. There is a window of a few nano-seconds > where a thread executing in parallel on another core might be > able to set the localbase variable (between the test for NULL > in this function and the assignment to it). The value that will > be returned by either thread will be identical, so there is no > risk of corruption of the result. But if I call getlocalbase, then set it via sysctl, then call getlocalbase again, I see the old value. If, however, I omit the first getlocalbase, I see the new value. This differs from how getenv/setenv of the value work, where you always see the up-to-date value. Maybe you think that's a feature, but it's something to watch out for and explicitly call out in the documentation. You also misunderstand all the subtleties of multithreading here. There are no acquire/release pairs so it is entirely legal for Thread 2 to read Thread 1's initialised value for localbase before the contents of it are visible (i.e. the pointer is initialised but the data is garbage). The `(volatile const char*)localbase` cast is also a complete waste of time. You probably meant to write `(const char * volatile)localbase` but even then that does nothing useful as the cast is too late. What you really were trying to write was `*(const char * volatile *)&localbase`, but you need proper atomics anyway for this to be safe. > This unlikely case may actually leak a heap allocated string > of typically tens of bytes (but with negligible probability). > > But this really is a non-issue, since there should never be a > reason to invoke this function in a multi-threaded context. Why not? There could easily be code out there calling getenv in a multi-threaded context so this is inadequate as a replacement. Yes it's inefficient but it's perfectly legal and imaginable. Also if malloc returns NULL I would quite like that to be an error for the function and not silently fall back on _PATH_LOCALBASE. > The result should be constant for the duration of execution > of the process (expect severe inconsistencies if that was not > the case) and all programs in base that are candidates for the > use of this function are non-threaded (and if they were multi- > threaded, then I'd expect this call to occur during start-up > of the program before any further threads are created). > > So, this is a non-issue and the comment tries to explain it. > Did I fail to make this clear in the comment? Maybe I should > have written "could use atomic ops" instead? > > Use of atomics or locks could prevent the race-condition. But > since I do not expect this function to be called from within > threads (it just doesn't make sense), the tiny time window of > a few nano-seconds which might lead to a double assignment to > the target variable (with one pointer value being lost), and > the worst case loss of 1KB of heap space in that case (more > likely 10 to 20 bytes rounded up to a 16 or 32 byte chunk), I > do not consider the complexities of either a lock or atomic ops > to be justified. > > Regards, STefan > _______________________________________________ svn-src-all@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
