execve (nr 59)

Linux Signature

int execve(const char *pathname, char *const argv[], char *const envp[]);

Description

Replaces the current process image with a new ELF binary. On success, the calling process’s address space, stack, and brk are replaced; the process continues execution at the new program’s entry point. On failure, the original process is unchanged.

Current Implementation

1. Copy arguments from userspace: Reads pathname (null-terminated string), argv (NULL-terminated array of string pointers), and envp (NULL-terminated array of string pointers) into kernel buffers before destroying the address space.
2. Resolve path: Resolves relative to the process’s cwd.
3. Read ELF from VFS: Loads the entire ELF binary via devices::vfs::read_file().
4. Parse ELF: Extracts PT_LOAD segments, entry point, and program headers via libkernel::elf::parse.
5. Create fresh PML4: Allocates a new user page table (kernel entries 256–510 are copied from the active PML4). The old PML4 and its user-half page tables are freed after switching CR3 (skipped for CLONE_VM shared PML4s).
6. Map ELF segments: Maps each PT_LOAD segment into the new PML4 with correct permissions (R/W/X).
7. Map user stack: 8 pages (32 KiB) at 0x0000_7FFF_F000_0000.
8. Build initial stack: Writes argc, argv pointers, envp pointers, and auxiliary vector (AT_PHDR, AT_PHENT, AT_PHNUM, AT_PAGESZ, AT_ENTRY, AT_UID, AT_RANDOM) onto the user stack.
9. Update process: Sets new pml4_phys, entry_point, user_stack_top, brk_base/brk_current, resets mmap_next/mmap_regions. Calls close_cloexec_fds() to close all file descriptors with FD_CLOEXEC set. Resets FS_BASE to 0 (new program’s libc will set up TLS).
10. Unblock vfork parent: If this process was created by clone(CLONE_VFORK), unblocks the parent thread.
11. Jump to userspace: Switches CR3 to the new PML4 and does iretq to the new entry point. Never returns.

On any error before step 9, returns a negative errno — the original process is unchanged.

Source: osl/src/exec.rs — sys_execve

Errors

Future Work

• Support #! (shebang) script execution.
• Proper AT_RANDOM with real randomness instead of a fixed address.