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arch: arm: aarch32: no PSLIM clearing in z_arm_userspace_enter()

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Modifying the PSP via an MSR instruction is not subject to
stack limit checking so we can remove the relevant code
block in the begining of z_arm_userspace_enter(), which clears
PSPLIM. We add a comment when setting the PSP to the privilege
stack to stress that clearing the PSPLIM is not required and it
is always a safe operation.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
This commit is contained in:
Ioannis Glaropoulos 2020-02-11 16:49:38 +01:00 • committed by Andrew Boie
commit b09607dee5
1 changed files with 14 additions and 7 deletions
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21
arch/arm/core/aarch32/userspace.S
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@ -43,12 +43,6 @@ SECTION_FUNC(TEXT,z_arm_userspace_enter)
/* move user_entry to lr */
mov lr, r0
#if defined(CONFIG_BUILTIN_STACK_GUARD)
/* clear stack pointer limit before setting the PSP */
mov r0, #0
msr PSPLIM, r0
#endif
/* prepare to set stack to privileged stack */
ldr r0, =_kernel
ldr r0, [r0, #_kernel_offset_to_current]
@ -73,10 +67,23 @@ SECTION_FUNC(TEXT,z_arm_userspace_enter)
*/
mov ip, sp
/* set stack to privileged stack */
/* set stack to privileged stack
*
* Note [applies only when CONFIG_BUILTIN_STACK_GUARD is enabled]:
* modifying PSP via MSR instruction is not subject to stack limit
* checking, so we do not need to clear PSPLIM before setting PSP.
* The operation is safe since, by design, the privileged stack is
* located in memory higher than the default (user) thread stack.
*/
msr PSP, r0
#if defined(CONFIG_BUILTIN_STACK_GUARD)
/* At this point the privileged stack is not yet protected by PSPLIM.
* Since we have just switched to the top of the privileged stack, we
* are safe, as long as the stack can accommodate the maximum exception
* stack frame.
*/
/* set stack pointer limit to the start of the priv stack */
ldr r0, =_kernel
ldr r0, [r0, #_kernel_offset_to_current]