race condition fix: block all signals before decrementing thread count

the existence of a (kernelspace) thread must never have observable
effects after the thread count is decremented. if signals are not
blocked, it could end up handling the signal for rsyscall and
contributing towards the count of threads which have changed ids,
causing a thread to be missed. this could lead to one thread retaining
unwanted privilege level.

this change may also address other subtle race conditions in
application code that uses signals.

diff --git a/src/thread/i386/__unmapself.s b/src/thread/i386/__unmapself.s
index 5c67496..bcf6e7f 100644 (file)
--- a/src/thread/i386/__unmapself.s
+++ b/src/thread/i386/__unmapself.s
@@ -2,15 +2,6 @@
 .global __unmapself
 .type   __unmapself,%function
 __unmapself:
-       call 1f
-       .long -1
-       .long -1
-1:     popl %ecx
-       xorl %ebx,%ebx
-       xorl %edx,%edx
-       movl $8,%esi
-       movl $175,%eax
-       int $128
        movl $91,%eax
        movl 4(%esp),%ebx
        movl 8(%esp),%ecx

diff --git a/src/thread/pthread_create.c b/src/thread/pthread_create.c
index 7c43695..c73c521 100644 (file)
--- a/src/thread/pthread_create.c
+++ b/src/thread/pthread_create.c
@@ -25,6 +25,8 @@ void __pthread_unwind_next(struct __ptcb *cb)
                }
        }
 
+       syscall4(__NR_sigprocmask, SIG_BLOCK, (long)(uint64_t[1]){-1}, 0, 8);
+
        if (!a_fetch_add(&libc.threads_minus_1, -1))
                exit(0);
 

diff --git a/src/thread/x86_64/__unmapself.s b/src/thread/x86_64/__unmapself.s
index 59092ea..d36257d 100644 (file)
--- a/src/thread/x86_64/__unmapself.s
+++ b/src/thread/x86_64/__unmapself.s
@@ -3,19 +3,6 @@
 .global __unmapself
 .type   __unmapself,%function
 __unmapself:
-       call 1f         /* glibc ABI compat */
-       .long -1
-       .long -1
-1:      push %rsi       /* save arg2 for munmap */
-       push %rdx       /* save arg3 for munmap */
-       mov %rdi,%rsi   /* rt_sigprocmask() args: move arg1 to rsi */
-       xor %rdi,%rdi
-       xor %rdx,%rdx
-       movq $8,%r10
-       movl $14,%eax   /* __NR_rt_sigprocmask */
-       syscall         /* call rt_sigprocmask(0,arg1,0,8) */
-       pop %rsi        /* munmap() args: reload from stack */
-       pop %rdi
        movl $11,%eax   /* __NR_munmap */
        syscall         /* munmap(arg2,arg3) */
        xor %rdi,%rdi   /* exit() args: always return success */