Identical to riscv64.
this code dates back to the original commit of the sh port, with no real clue as to how the bug was introduced. it looks like it was written to assume the return address was pushed to the stack like on x86, rather than arriving in the pr special register.
Author: Xiaojuan Zhai <zhaixiaojuan@loongson.cn> Author: Meidan Li <limeidan@loongson.cn> Author: Guoqi Chen <chenguoqi@loongson.cn> Author: Xiaolin Zhao <zhaoxiaolin@loongson.cn> Author: Fan peng <fanpeng@loongson.cn> Author: Jiantao Shan <shanjiantao@loongson.cn> Author: Xuhui Qiang <qiangxuhui@loongson.cn> Author: Jingyun Hua <huajingyun@loongson.cn> Author: Liu xue <liuxue@loongson.cn> Author: Hongliang Wang <wanghongliang@loongson.cn>
ever since commit 8f11e6127fe93093f81a52b15bb1537edc3fc8af introduced the thread list lock, this has been wrong. initially, it was wrong via calling free from the context with the thread list lock held. commit aa5a9d15e09851f7b4a1668e9dbde0f6234abada deferred the unsafe free but added a lock, which was also unsafe. in particular, it could deadlock if code holding freebuf_queue_lock was interrupted by a signal handler that takes the thread list lock. commit 4d5aa20a94a2d3fae3e69289dc23ecafbd0c16c4 observed that there was a lock here but failed to notice that it's invalid. there is no easy solution to this problem with locks; any attempt at solving it while still using locks would require the lock to be an AS-safe one (blocking signals on each access to the dlerror buffer list to check if there's deferred free work to be done) which would be excessively costly, and there are also lock order considerations with respect to how the lock would be handled at fork. instead, just use an atomic list.
dl_iterate_phdr was wrongly reporting the address of the DSO's PT_TLS image rather than the calling thread's instance of the TLS. the man page, which is essentially normative for a nonstandard function of this sort, clearly specifies the latter. it does not clarify where exactly within/relative-to the image the pointer should point, but the reasonable thing to do is match the ABI's DTP offset, and this seems to be what other implementations do.
as the outcome of Austin Group tracker issue #62, future editions of POSIX have dropped the requirement that fork be AS-safe. this allows but does not require implementations to synchronize fork with internal locks and give forked children of multithreaded parents a partly or fully unrestricted execution environment where they can continue to use the standard library (per POSIX, they can only portably use AS-safe functions). up until recently, taking this allowance did not seem desirable. however, commit 8ed2bd8bfcb4ea6448afb55a941f4b5b2b0398c0 exposed the extent to which applications and libraries are depending on the ability to use malloc and other non-AS-safe interfaces in MT-forked children, by converting latent very-low-probability catastrophic state corruption into predictable deadlock. dealing with the fallout has been a huge burden for users/distros. while it looks like most of the non-portable usage in applications could be fixed given sufficient effort, at least some of it seems to occur in language runtimes which are exposing the ability to run unrestricted code in the child as part of the contract with the programmer. any attempt at fixing such contracts is not just a technical problem but a social one, and is probably not tractable. this patch extends the fork function to take locks for all libc singletons in the parent, and release or reset those locks in the child, so that when the underlying fork operation takes place, the state protected by these locks is consistent and ready for the child to use. locking is skipped in the case where the parent is single-threaded so as not to interfere with legacy AS-safety property of fork in single-threaded programs. lock order is mostly arbitrary, but the malloc locks (including bump allocator in case it's used) must be taken after the locks on any subsystems that might use malloc, and non-AS-safe locks cannot be taken while the thread list lock is held, imposing a requirement that it be taken last.
this change lifts undocumented restrictions on calls by replacement mallocs to libc functions that might take these locks, and sets the stage for lifting restrictions on the child execution environment after multithreaded fork. care is taken to #define macros to replace all four functions (malloc, calloc, realloc, free) even if not all of them will be used, using an undefined symbol name for the ones intended not to be used so that any inadvertent future use will be caught at compile time rather than directed to the wrong implementation.
thread-local buffers allocated for dlerror need to be queued for free at a later time when the owning thread exits, since malloc may be replaced by application code and the exiting context is not valid to call application code from. the code to process queue of pending frees, introduced in commit aa5a9d15e09851f7b4a1668e9dbde0f6234abada, gratuitously held the lock for the entire duration of queue processing, updating the global queue pointer after each free, despite there being no logical requirement that all frees finish before another thread can access the queue. instead, immediately claim the whole queue for freeing and release the lock, then walk the list and perform frees without the lock held. the change is unlikely to make any meaningful difference to performance, but it eliminates one point where the allocator is called under an internal lock. since the allocator may be application-provided, such calls are undesirable because they allow application code to impede forward progress of libc functions in other threads arbitrarily long, and to induce deadlock if it calls a libc function that requires the same lock. the change also eliminates a lock ordering consideration that's an impediment upcoming work with multithreaded fork.