Subject: where/how arm start first jump from svc to user in kernel

From: [email-masked] (Mark Rutland)
Date: Tue, 21 Oct 2014 13:41:33 +0100

On Tue, Oct 21, 2014 at 12:37:02PM +0100, vichy wrote:
> hi Catalin:
> > As a quick answer, it's ret_to_user (or ret_slow_syscall to be more
> > precise) in arch/arm/kernel/entry-common.S.
> >
> > How it gets there is a bit more complicated. The initial kernel call
> > path:
> >
> > start_kernel()
> >   rest_init()
> >     kernel_thread(kernel_init)
> >
> > kernel_thread() creates a new thread which executes the kernel_init()
> > function. After the multitude of calls that kernel_thread() -> do_fork()
> > does, it eventually calls copy_thread() which sets the
> > thread->cpu_context.pc to ret_from_fork and the actual address of
> > kernel_init in thread->cpu_context.r5. When the kernel eventually
> > switches to the new kernel thread, it will jump to
> > thread->cpu_context.pc which is ret_from_fork (see __switch_to in
> > arch/arm/kernel/entry-armv.S). ret_from_fork() branches to kernel_init()
> > but sets the return address (LR) to label 1 in ret_from_fork.
> >
> > After kernel_init() does its work, it eventually calls
> > run_init_process() which invokes do_execve() and eventually
> > load_elf_binary(). If the ELF binary was successfully loaded, this
> > function calls start_thread() with the ELF entry point. The
> > start_thread() function populates the pt_regs structure on the stack so
> > that regs->ARM_pc points to the ELF entry point and regs->ARM_cpsr has
> > the user mode bits set.
> Many thanks for your kind and detail explanation.
> 
> >
> > Going back to kernel_init(), if run_init_process() was successful, it
> > returns to label 1 in ret_from_fork which branches to ret_slow_syscall
> > which eventually returns to user space via the restore_user_regs macro
> > (in arch/arm/kernel/entry-header.S).
> 
> Below is excerpted from arch/arm/kernel/entry-header.S
> 
>         .macro  restore_user_regs, fast = 0, offset = 0
>         ldr     r1, [sp, #\offset + S_PSR]      @ get calling cpsr
>         ldr     lr, [sp, #\offset + S_PC]!      @ get pc
>         msr     spsr_cxsf, r1                   @ save in spsr_svc
>         ..........
>         add     sp, sp, #S_FRAME_SIZE - S_PC
>         movs    pc, lr                          @ return & move
> spsr_svc into cpsr
>         .endm
> 
> so before armv8 architecture and kernel switch to user mode (arm svn-> user),
> we first put the cpsr, the mode we want to jump to, in current spsr.
> Then update pc counter to where the ELF entry, right?

Essentially yes, though the PC is not always the ELF entry address.
Consider returning from syscalls or interrupts.

> BTW, in armv8, aarch64, arch/arm64/entry.S, kernel_exit,
> (if I look at the right place.)
> 
> .macro  kernel_exit, el, ret = 0
>         ..............................
>         msr     elr_el1, x21                    // set up the return data
>         msr     spsr_el1, x22
>         .if     \el == 0
>         msr     sp_el0, x23
>         .endif
>         pop     x10, x11
>         ............................
>         ldr     lr, [sp], #S_FRAME_SIZE - S_LR  // load LR and restore SP
>         eret                                    // return to kernel
> .endm
> 
> Is there any special reason we use eret in v8 instead of modify pc in v7?
> I have looked arm v8 architecture reference menu but get no explanation so far.

There is no movs pc, lr equivalent in AArch64. The eret instruction is
the only mechanism for dropping to a lower privileged exception level
(e.g. to EL0 userspace from an EL1 kernel).

The A32 eret instruction was only added with the ARMv7 virtualization
extensions (and in PL1 behaves as movs pc, lr if present). So it's
necessary to us movs pc, lr to function across a wide variety of CPUs,
and there's no benefit to using eret.

Thanks,
Mark.