Initial commit for publication

1 files changed, 284 insertions, 0 deletions

diff --git a/lib/thread_create.asm b/lib/thread_create.asm
new file mode 100644
index 0000000..4163d29
--- /dev/null
+++ b/lib/thread_create.asm
@@ -0,0 +1,284 @@
+; MIT license:
+;
+; Copyright (c) 2023 Marcus R.A. Newman (prefetch@prefetch.eu)
+;
+; Permission is hereby granted, free of charge, to any person obtaining a copy
+; of this software and associated documentation files (the "Software"), to deal
+; in the Software without restriction, including without limitation the rights
+; to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+; copies of the Software, and to permit persons to whom the Software is
+; furnished to do so, subject to the following conditions:
+;
+; The above copyright notice and this permission notice shall be included in
+; all copies or substantial portions of the Software.
+;
+; THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+; AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+; LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+; OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+; SOFTWARE.
+
+
+; Cheat sheet for Linux' x86_64 calling convention:
+;
+; - free to overwrite; the caller should save them:
+;       rax, rcx, rdx, rsi, rdi, r8-r11, xmm0-xmm15
+; - caller expects no change; 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: 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_end ; Nowhere to store the thread handle
+		test rsi, rsi
+		jz create_end ; 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: settings 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_end
+
+		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+		;;;; 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_end
+
+		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+		;;;; 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_end      ; 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_end:
+		; Restore callee-save registers
+		pop rbx
+
+		ret
+