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Diffstat (limited to 'src/lock_release.asm')
| -rw-r--r-- | src/lock_release.asm | 132 |
1 files changed, 132 insertions, 0 deletions
diff --git a/src/lock_release.asm b/src/lock_release.asm new file mode 100644 index 0000000..f86caa2 --- /dev/null +++ b/src/lock_release.asm @@ -0,0 +1,132 @@ +; 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_GETTID 186 +%define SYS_FUTEX 202 + +; Relevant operations for futex +%define FUTEX_UNLOCK_PI 7 +%define FUTEX_PRIVATE_FLAG 0x80 + +; Relevant bits for futex dword +%define FUTEX_TID_MASK 0x3fffffff +%define FUTEX_OWNER_DIED 0x40000000 +%define FUTEX_WAITERS 0x80000000 + + +; Release an acquired lock if we're who acquired it. Argument: +; rdi: struct{u32,u32,u32}* = handle of lock to release +; Returns zero on success, or a standard error code. +global linen_lock_release +linen_lock_release: + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;; Check validity of argument ;;;; + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + + ; Return EINVAL if rdi is NULL or invalid + mov eax, -22 ; (EINVAL = -22) + + test rdi, rdi + jz release_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 lock handle! + ; To verify this we check the canary value stored at [rdi + 8]. + mov ecx, [rdi + 8] + cmp ecx, 0xCAFEBABE + jnz release_return + + ; Lock owners are identified by their TID; let's find ours. + ; The gettid system call simply returns our Linux thread ID. + ; See: man 2 gettid + + ; gettid: rax = system call ID + mov eax, SYS_GETTID + ; gettid: rax = gettid() + syscall + + ; Save a copy of our TID (no need for an error check) + mov edx, eax + + ; Return EPERM if this lock currently doesn't belong to us + mov eax, -1 ; (EPERM = -1) + + ; Read the futex dword at [rdi] and keep its lowest 30 bits + mov ecx, [rdi] + and ecx, FUTEX_TID_MASK + ; Those bits contain the owner's TID; it should be our TID + cmp ecx, edx + jne release_return + + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;; (Partially) release our lock ;;;; + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + + ; Decrement the recursion counter. If it's still > 1, we're done here. + dec dword [rdi + 4] + jnz release_success + ; If it reaches 0, it's time for a full release by setting [rdi] to 0. + + ; Restore our saved TID to eax for "lock cmpxchg" below + mov eax, edx + + ; Atomically try to set the dword at [rdi] to 0 if it was equal to our TID. + ; if ([rdi] == eax]) { [rdi] = 0; goto release_success; } else { eax = [rdi]; } + xor ecx, ecx + lock cmpxchg [rdi], ecx + je release_success + + ; We failed because [rdi] wasn't equal to our TID. In theory, + ; that can mean only one thing: [rdi] = (edx | FUTEX_WAITERS). + ; In that case we need to ask the kernel to wake up the threads + ; who are waiting (via a futex system call) for [rdi] to change. + ; See: man 2 futex + + ; futex: rdi = uaddr: address of the dword to announce for + ; futex: rsi = futex_op: which futex operation we want: + ; - FUTEX_UNLOCK_PI: wake up one thread sleeping via FUTEX_LOCK_PI + ; - FUTEX_PRIVATE_FLAG: this lock isn't shared with another process + mov esi, (FUTEX_UNLOCK_PI | FUTEX_PRIVATE_FLAG) ; futex: futex_op + ; futex: rdx = val: ignored when FUTEX_UNLOCK_PI is used + ; futex: r10 = timeout: ignored when FUTEX_UNLOCK_PI is used + ; futex: r8 = uaddr2: ignored when FUTEX_UNLOCK_PI is used + ; futex: r9 = val3: ignored when FUTEX_UNLOCK_PI is used + ; futex: rax = system call ID + mov eax, SYS_FUTEX + ; futex: rax = futex(rdi, rsi, (rdx), (r10), (r8), (r9)) + syscall + + ; Check result of futex: nonzero means failure + test rax, rax + jnz release_return + + release_success: + xor eax, eax + + release_return: + ret |