814629dd64
Upon successful completion, the execve() system call invokes exec_setregs() to initialize the registers of the initial thread of the newly executed process. What is weird is that when execve() returns, it still goes through the normal system call return path, clobbering the registers with the system call's return value (td->td_retval). Though this doesn't seem to be problematic for x86 most of the times (as the value of eax/rax doesn't matter upon startup), this can be pretty frustrating for architectures where function argument and return registers overlap (e.g., ARM). On these systems, exec_setregs() also needs to initialize td_retval. Even worse are architectures where cpu_set_syscall_retval() sets registers to values not derived from td_retval. On these architectures, there is no way cpu_set_syscall_retval() can set registers to the way it wants them to be upon the start of execution. To get rid of this madness, let sys_execve() return EJUSTRETURN. This will cause cpu_set_syscall_retval() to leave registers intact. This makes process execution easier to understand. It also eliminates the difference between execution of the initial process and successive ones. The initial call to sys_execve() is not performed through a system call context. Reviewed by: kib, jhibbits Differential Revision: https://reviews.freebsd.org/D13180 |
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