File lib/libc/sys/swapcontext.c has looks to have BSD-2-Clause but second clausal is from BSD-3-Clause or BSD-4-Clause license text:
* 2. Neither the name of the author nor the names of its contributors * may be used to endorse or promote products derived from this software * without specific prior written permission.
And not the normal BSD-2-Clause license. There is several files with this kind of modification and many of files are tagged with BSD-2-Clause.
lib/libc/gen/_thread_init.c lib/libc/gen/elf_utils.c lib/libc/sys/swapcontext.c lib/libc/i386/gen/_ctx_start.S lib/libc/i386/gen/makecontext.c lib/libsys/amd64/_umtx_op_err.S lib/libsys/i386/_umtx_op_err.S lib/libthr/thread/thr_ctrdtr.c lib/libthr/thread/thr_event.c lib/libthr/thread/thr_fork.c lib/libthr/arch/amd64/include/pthread_md.h
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