https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=9580d7f3792f6635382ef8220cfbea802be8dd5e
commit 9580d7f3792f6635382ef8220cfbea802be8dd5e Author: Takashi Yano <[email protected]> Date: Wed Dec 24 03:14:03 2025 +0900 Cygwin: thread: Fix stack alignment for PTHREAD_CANCEL_ASYNCHRONOUS The test case winsup/testsuites/winsup.api/pthread/cancel2 fails on Windows 11 and Windows Server 2025, while it works on Windows 10 and Windows Server 2022. PTHREAD_CANCEL_ASYNCHRONOUS is implemented by forcing the target thread's instruction pointer (IP) to pthread:: static_cancel_self() using [GS]etThreadContext(). static_cancel_self() will call Windows API function during thread shutdown. A misaligned stack will lead to unexpected exceptions. Previously, the stack pointer was not maintained to 16-byte alignment, even though this is required by 64-bit Windows ABI. At the start of the function prologue, the stack is expected to be at an offset of 8 byte from 16-byte boundary (SP % 16 == 8) in x86_64 architecture, as the call instruction has just pushed the return IP onto the stack. However, this appears to have been overlooked when cygwin first added x86_64 support. This patch fixes this issue by aligning the stack pointer as well as the instruction pointer in the PTHREAD_CANCEL_ASYNCHRONOUS handling. Addresses: https://cygwin.com/pipermail/cygwin/2025-December/259117.html Fixes: 61522196c715 ("* Merge in cygwin-64bit-branch.") Reported-by: Takashi Yano <[email protected]> Reviewed-by: Jon Turney <[email protected]> Signed-off-by: Takashi Yano <[email protected]> (cherry picked from commit b41e3b652c4215556a62a2f47e8f8f11553fa550) Diff: --- winsup/cygwin/thread.cc | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/winsup/cygwin/thread.cc b/winsup/cygwin/thread.cc index 9ee96504b..d410b9575 100644 --- a/winsup/cygwin/thread.cc +++ b/winsup/cygwin/thread.cc @@ -629,7 +629,27 @@ pthread::cancel () threadlist_t *tl_entry = cygheap->find_tls (cygtls); if (!cygtls->inside_kernel (&context)) { +#if defined(__x86_64__) + /* Need to maintain the alignment of the stack pointer. + https://learn.microsoft.com/en-us/cpp/build/stack-usage?view=msvc-170 + states, + "The stack will always be maintained 16-byte aligned, + except within the prolog (for example, after the return + address is pushed),", + that is, we need 16n + 8 byte alignment here because the stack + pointer must be maintaiined to the same alignment required by + the function prologue. Since the call instruction pushes the + return address (rip) onto the stack, which is 8 bytes, + an additional 8 bytes is required to emulate this behaviour. + However, we do not need to push return address itself, because + pthread::static_cancel_self() must not return. */ + context.Rsp &= ~0x07UL; + if ((context.Rsp & 8) == 0) + context.Rsp -= 8; context.Rip = (ULONG_PTR) pthread::static_cancel_self; +#else +#error unimplemented for this target +#endif SetThreadContext (win32_obj_id, &context); } cygheap->unlock_tls (tl_entry);
