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Diffstat (limited to 'src/thread_create.asm')
| -rw-r--r-- | src/thread_create.asm | 264 |
1 files changed, 264 insertions, 0 deletions
diff --git a/src/thread_create.asm b/src/thread_create.asm new file mode 100644 index 0000000..9a6fe78 --- /dev/null +++ b/src/thread_create.asm @@ -0,0 +1,264 @@ +; 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_MMAP 9 +%define SYS_MPROTECT 10 +%define SYS_CLONE 56 +%define SYS_EXIT 60 + +; Relevant flags for mmap +%define MAP_SHARED 0x00001 +%define MAP_PRIVATE 0x00002 +%define MAP_ANONYMOUS 0x00020 +;%define MAP_GROWSDOWN 0x00100 ; Insecure, segfaults anyway +%define MAP_LOCKED 0x02000 +%define MAP_POPULATE 0x08000 +%define MAP_STACK 0x20000 + +; Relevant flags for mprotect +%define PROT_READ 0x1 +%define PROT_WRITE 0x2 + +; Relevant flags for clone +%define CLONE_VM 0x00000100 +%define CLONE_FS 0x00000200 +%define CLONE_FILES 0x00000400 +%define CLONE_SIGHAND 0x00000800 +%define CLONE_PARENT 0x00008000 +%define CLONE_THREAD 0x00010000 +%define CLONE_SYSVSEM 0x00040000 +%define CLONE_SETTLS 0x00080000 +%define CLONE_PARENT_SETTID 0x00100000 +%define CLONE_CHILD_CLEARTID 0x00200000 +%define CLONE_CHILD_SETTID 0x01000000 +%define CLONE_IO 0x80000000 + + +%define STACK_SIZE 2097152 ; 2 MiB stack +%define GUARD_PAGE 4096 ; 4 KiB guard page + + +; Create a new thread executing a given function. Arguments: +; rdi: struct{u32,u32}** = where to put the thread handle +; rsi: void* (*)(void*) = function to make the child run +; rdx: void* = single argument for function +; Returns zero on success, or a standard error code. +global linen_thread_create +linen_thread_create: + ; Callee-save registers + push rbx + + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;; Check validity of arguments ;;;; + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + + ; Return EINVAL if any argument is NULL + mov eax, -22 ; (EINVAL = -22) + test rdi, rdi + jz create_return ; Nowhere to store the thread handle + test rsi, rsi + jz create_return ; No function for the thread to run + + ; Note: we allow rdx to be NULL; in that case the worst that can happen + ; is a segmentation fault in the user's code (not really our problem). + + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;; Allocate a stack and guard page ;;;; + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + + ; Save these registers: we'll clobber them for the mmap call + mov rbx, rdi + push rdx + push rsi + + ; The mmap system call does many things, in this case allocate memory. + ; See: man 2 mmap + + ; mmap: rdi = addr: address for mapping; 0 lets kernel choose + xor edi, edi + ; mmap: rsi = length: size of buffer to allocate + mov esi, (STACK_SIZE + GUARD_PAGE) + ; mmap: rdx = prot: mprotect-style access permissions + mov edx, (PROT_WRITE | PROT_READ) + ; mmap: r10 = flags: configuration flags for mapping: + ; - MAP_ANONYMOUS: there is no file backing this buffer + ; - MAP_PRIVATE: only this process can see thread's stack + ; - MAP_STACK: no-op; inform kernel that this is a stack + mov r10, (MAP_ANONYMOUS | MAP_PRIVATE | MAP_STACK) + ; mmap: r8 = fd: ignored for MAP_ANONYMOUS, recommended -1 + mov r8, -1 + ; mmap: r9 = offset: should be 0 when MAP_ANONYMOUS is used + xor r9, r9 + ; mmap: rax = system call ID + mov eax, SYS_MMAP + ; mmap: rax = mmap(rdi, rsi, rdx, r10, r8, 9) + syscall + + ; Pop these now before we start branching. Those registers + ; won't be used by the next system calls, so they're safe. + pop r8 ; function + pop r9 ; argument + + ; Check result of mmap: negative means failure, + ; otherwise rax is the address of the new mapping. + test rax, rax + js create_return + + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;; Revoke guard page's R/W permissions ;;;; + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + + ; Keep in mind that stacks grow downward, so the guard page is at + ; the lowest address of the newly-allocated buffer, i.e. at [rax]. + + ; The mprotect system call changes the permissions of a memory region. + ; See: man 2 mprotect + + ; mprotect: rdi = addr: lower address of region to control + mov rdi, rax + ; mprotect: rsi = len: size of region, one page in this case + mov esi, GUARD_PAGE + ; mprotect: rdx = prot: access permissions; zero for none + xor edx, edx + ; mprotect: rax = system call ID + mov eax, SYS_MPROTECT + ; mprotect: rax = mprotect(rdi, rsi, rdx) + syscall + + ; Check result of mprotect: nonzero means failure + test rax, rax + jnz create_return + + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;; Spawn a thread with the new stack ;;;; + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + + ; The clone system call spawns a new thread, cloned from a parent. + ; Both threads end up running the same code, i.e. it returns "twice", + ; once in the parent (0 if success) and once in the child (the TID). + ; See: man 2 clone + + ; clone: rsi = stack + ; Currently rdi points to the lowest byte of the stack area. + ; Again, stacks grow downward, so we calculate the address of + ; the top qword to use as the child thread's starting point. + lea rsi, [rdi + (STACK_SIZE + GUARD_PAGE - 8)] + + ; clone: rdi = flags: settings for cloned thread + ; These flags make the parent and child share resources: + ; - CLONE_VM: memory address space + ; - CLONE_FS: filesystem information, e.g. working directory + ; - CLONE_FILES: file descriptor table + ; - CLONE_IO: I/O scheduler context + ; - CLONE_SIGHAND: signal handlers + ; - CLONE_PARENT: parent process (implied by CLONE_THREAD?) + ; - CLONE_THREAD: shared PID, distinguish by TID instead (I think?) + ; These flags are relevant for a threading API: + ; - CLONE_CHILD_SETTID: store child's TID at supplied address (in r10) + ; - CLONE_CHILD_CLEARTID: set stored TID to zero when child finishes + ; (this will be used for joining threads) + mov edi, (CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_IO \ + | CLONE_SIGHAND | CLONE_PARENT | CLONE_THREAD \ + | CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID) + + ; clone: rdx = parent_tid: ignored unless CLONE_PARENT_SETTID is used + + ; clone: r10 = child_tid: address to store new thread's TID + ; We use "bottom" of stack (rsi), i.e. where child will start. + mov r10, rsi + + ; clone: r8 = tls: ignored unless CLONE_SETTLS is used + + ; clone: rax = system call ID + mov eax, SYS_CLONE + ; clone: rax = clone(rdi, rsi, (rdx), r10, (r8)); + syscall + + ; Ideally, both parent and new-born child are executing this code now. + + ; Check result of clone: + test rax, rax + js create_return ; Negative means failure + jnz create_success ; Positive means we're in the parent thread + ; Zero means we're in the child thread + + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;; Initialization in child thread ;;;; + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + + ; Best practice is to clear the frame pointer + xor ebp, ebp + + ; Move argument into place and call supplied function + mov rdi, r9 + call r8 + + ; Once done, leave function's return value lying around + push rax + + ; Exit the thread with return value 0 + xor edi, edi + mov rax, SYS_EXIT + syscall ; (never returns) + + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + ;;;; Clean up in parent thread ;;;; + ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; + + create_success: + ; We use the highest dword of the child's stack buffer as a futex + ; to detect when it has finished (see CLONE_CHILD_CLEARTID above). + ; That dword's address also acts as a thread handle for our API, + ; so we store it at the address the caller supplied (now in rbx). + mov [rbx], rsi + + ; We place a canary value in the unused dword at the top: + ; checking this value tells us if a thread handle is valid. + mov dword [rsi + 4], 0xDEADBEEF + + ; "Sketch" of child's stack buffer's layout: + ; + ; (bottom of range allocated by mmap) + ; 4 KiB: guard page, unused + ; (bottom of usable buffer) + ; ... + ; ... Child is currently doing work here ... + ; ... + ; qword: return address of function called by child (from r8) + ; dword: futex to detect when child has returned (address: rsi) + ; dword: canary value to know if handle is valid (address: rsi + 4) + ; (top of range allocated by mmap = top of usable buffer) + + ; Return 0 for success + xor eax, eax + + create_return: + ; Restore callee-save registers + pop rbx + + ret + |