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Linux 6.11 has getrandom() in vDSO. It operates on a thread-local opaque state allocated with mmap using flags specified by the vDSO. Multiple states are allocated at once, as many as fit into a page, and these are held in an array of available states to be doled out to each thread upon first use, and recycled when a thread terminates. As these states run low, more are allocated. To make this procedure async-signal-safe, a simple guard is used in the LSB of the opaque state address, falling back to the syscall if there's reentrancy contention. Also, _Fork() is handled by blocking signals on opaque state allocation (so _Fork() always sees a consistent state even if it interrupts a getrandom() call) and by iterating over the thread stack cache on reclaim_stack. Each opaque state will be in the free states list (grnd_alloc.states) or allocated to a running thread. The cancellation is handled by always using GRND_NONBLOCK flags while calling the vDSO, and falling back to the cancellable syscall if the kernel returns EAGAIN (would block). Since getrandom is not defined by POSIX and cancellation is supported as an extension, the cancellation is handled as 'may occur' instead of 'shall occur' [1], meaning that if vDSO does not block (the expected behavior) getrandom will not act as a cancellation entrypoint. It avoids a pthread_testcancel call on the fast path (different than 'shall occur' functions, like sem_wait()). It is currently enabled for x86_64, which is available in Linux 6.11, and aarch64, powerpc32, powerpc64, loongarch64, and s390x, which are available in Linux 6.12. Link: https://pubs.opengroup.org/onlinepubs/9799919799/nframe.html [1] Co-developed-by: Jason A. Donenfeld <Jason@zx2c4.com> Tested-by: Jason A. Donenfeld <Jason@zx2c4.com> # x86_64 Tested-by: Adhemerval Zanella <adhemerval.zanella@linaro.org> # x86_64, aarch64 Tested-by: Xi Ruoyao <xry111@xry111.site> # x86_64, aarch64, loongarch64 Tested-by: Stefan Liebler <stli@linux.ibm.com> # s390x |
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| .. | ||
| bits | ||
| lp64 | ||
| sys | ||
| arch-syscall.h | ||
| atomic-machine.h | ||
| clone3.S | ||
| clone.S | ||
| configure | ||
| configure.ac | ||
| cpu-features.c | ||
| cpu-features.h | ||
| dl-cache.h | ||
| dl-procinfo.c | ||
| dl-sysdep.c | ||
| fixup-asm-unistd.h | ||
| getcontext.S | ||
| gettimeofday.c | ||
| Implies | ||
| ldd-rewrite.sed | ||
| libc-start.c | ||
| localplt.data | ||
| makecontext.c | ||
| Makefile | ||
| pointer_guard.h | ||
| readelflib.c | ||
| setcontext.S | ||
| shlib-versions | ||
| sigcontextinfo.h | ||
| swapcontext.S | ||
| syscall_cancel.S | ||
| sysdep.h | ||
| sysdep.S | ||
| ucontext_i.sym | ||
| ucontext-macros.h | ||
| vfork.S | ||