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Diffstat (limited to 'src/thread_finish.asm')
| -rw-r--r-- | src/thread_finish.asm | 147 |
1 files changed, 147 insertions, 0 deletions
diff --git a/src/thread_finish.asm b/src/thread_finish.asm new file mode 100644 index 0000000..860b0a4 --- /dev/null +++ b/src/thread_finish.asm @@ -0,0 +1,147 @@ +; Under MIT license, see /LICENSE.txt + + +; Cheat sheet for Linux' x86_64 calling convention: +; +; - free to overwrite (caller should save them): +; rax, rcx, rdx, rsi, rdi, r8-r11, xmm0-xmm15 +; - caller expects be kept (callee should save them): +; rbx, rbp, r12-r15 +; +; - for passing paramters to functions: +; rdi, rsi, rdx, rcx, r8, r9, xmm0-xmm7 +; - for getting return values from functions: +; rax, rdx, xmm0 +; +; - for passing parameters to syscalls: +; rax, rdi, rsi, rdx, r10, r8, r9 +; - for getting return values from syscalls: +; rax, rdx +; - overwritten by syscalls (all others preserved): +; rcx, r11 + + +section .text + + +; Relevant system call IDs +%define SYS_MUNMAP 11 +%define SYS_FUTEX 202 + +; Relevant operations for futex +%define FUTEX_WAIT 0 +%define FUTEX_PRIVATE_FLAG 0x80 + + +%define STACK_SIZE 2097152 ; 2 MiB stack +%define GUARD_PAGE 4096 ; 4 KiB guard page + + +; Wait for thread to exit, save its return value, and clean up. Arguments: +; rdi: struct{u32,u32}* = handle of the thread to wait for +; rsi: void** = where to put void* returned by thread +; Returns zero on success, or a standard error code. +global linen_thread_finish +linen_thread_finish: + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;; Check validity of arguments ;;;; + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + + ; Return EINVAL if rdi is NULL or otherwise invalid + mov eax, -22 ; (EINVAL = -22) + + test rdi, rdi + jz finish_return ; rdi is NULL + + ; rdi is nonzero, so let's just assume it's a valid pointer; + ; if that assumption is wrong we'll get a segmentation fault. + ; But we don't yet trust that [rdi] is a valid thread handle! + ; To verify this we check the canary value stored at [rdi + 4]. + mov ecx, [rdi + 4] + cmp ecx, 0xDEADBEEF + jnz finish_return + + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;; Wait until thread is finished ;;;; + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + + ; We'll clobber rsi if we need to set up a futex call + mov r8, rsi + + finish_retry: + ; When spawning, we set CLONE_CHILD_SETTID and CLONE_CHILD_CLEARTID: + ; [rdi] contains the child thread's TID, and will get automatically + ; cleared (to 0) when the child exits; this is what we'll watch for. + + ; Atomically check whether the target thread is still running. + ; if ([rdi] == 0) { goto finish_success; } else { eax = [rdi]; } + xor eax, eax + lock cmpxchg [rdi], eax + jz finish_success + + ; The thread is still busy, so block until it's done. + ; The futex system call waits until the dword at an + ; address (rdi) deviates from an expected value (eax). + ; See: man 2 futex + + ; futex: rdi = uaddr: address of the dword to watch + ; futex: rsi = futex_op: which futex operation we want: + ; - FUTEX_WAIT: block until the value at [rdi] changes + ; - FUTEX_PRIVATE_FLAG: FIXME waits forever, I don't understand why + mov esi, FUTEX_WAIT + ; futex: rdx = val: the expected value at [rdi] before it changes + mov edx, eax + ; futex: r10 = timeout: in case we had a deadline (we don't) + xor r10, r10 + ; futex: r8 = uaddr2: ignored when FUTEX_WAIT is used + ; futex: r9 = val3: ignored when FUTEX_WAIT is used + ; futex: rax = system call ID + mov eax, SYS_FUTEX + ; futex: rax = futex(rdi, rsi, rdx, r10, (r8), (r9)) + syscall + + ; Sometimes the thread exits after the "lock cmpxchg" instruction + ; but before the futex call. In that case, futex returns EAGAIN. + cmp rax, -11 ; (EAGAIN = -11) + je finish_retry + + ; Any other nonzero return value means failure + test rax, rax + jnz finish_return + + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;; Clean up after thread's exit ;;;; + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + + finish_success: + ; The thread left its function return value on the stack, read it + mov rdx, [rdi - 8] + + ; The munmap system call destroys mappings created by mmap. + ; In this case that means deallocating the stack buffer. + ; See: man 2 munmap + + ; munmap: rdi = addr: lowest address of region to unmap + ; Our rdi is near the buffer's top, so we must subtract + sub rdi, (STACK_SIZE + GUARD_PAGE - 8) + ; munmap: rsi = length: size of region starting from rdi + mov esi, (STACK_SIZE + GUARD_PAGE) + ; munmap: rax = system call ID + mov eax, SYS_MUNMAP + ; munmap: rax = munmap(rdi, rsi) + syscall + + ; Check result of munmap: nonzero means failure + test rax, rax + jnz finish_return + + ; Check if caller gave a location (r8) to save the return value (rdx) + test r8, r8 + jz finish_return ; caller doesn't care: gave NULL pointer + mov [r8], rdx + ; Note: if munmap failed, the buffer is still there, so we + ; can safely return an error without losing the return value. + + finish_return: + ret + |