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riscv_fork.c: Fix race condition when handling parent integer registers 

We need to record the parent's integer register context upon exception
entry to a separate non-volatile area. Why?

Because xcp.regs can move due to a context switch within the fork() system
call, be it either via interrupt or a synchronization point.

Fix this by adding a "sregs" area where the saved user context is placed.
The critical section within fork() is also unnecessary.
This commit is contained in:
Ville Juven 2024-09-26 13:03:12 +03:00 committed by Xiang Xiao
parent 172d2a8491
commit 9ef76e3735
3 changed files with 12 additions and 13 deletions
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6
arch/risc-v/include/irq.h
View File

@ -613,6 +613,12 @@ struct xcptcontext
uintreg_t *regs;
#ifdef CONFIG_LIB_SYSCALL
/* User integer registers upon system call entry */
uintreg_t *sregs;
#endif
/* FPU register save area */
#if defined(CONFIG_ARCH_FPU) && defined(CONFIG_ARCH_LAZYFPU)

17
arch/risc-v/src/common/riscv_fork.c
View File

@ -108,19 +108,14 @@ pid_t riscv_fork(const struct fork_s *context)
uintptr_t newtop;
uintptr_t stacktop;
uintptr_t stackutil;
irqstate_t flags;
#ifdef CONFIG_SCHED_THREAD_LOCAL
uintptr_t tp;
#endif
UNUSED(context);
/* parent regs may change in irq, we should disable irq here */
flags = up_irq_save();
/* Allocate and initialize a TCB for the child task. */
child = nxtask_setup_fork((start_t)parent->xcp.regs[REG_RA]);
child = nxtask_setup_fork((start_t)parent->xcp.sregs[REG_RA]);
if (!child)
{
sinfo("nxtask_setup_fork failed\n");
@ -130,15 +125,15 @@ pid_t riscv_fork(const struct fork_s *context)
/* Copy parent user stack to child */
stacktop = (uintptr_t)parent->stack_base_ptr + parent->adj_stack_size;
DEBUGASSERT(stacktop > parent->xcp.regs[REG_SP]);
stackutil = stacktop - parent->xcp.regs[REG_SP];
DEBUGASSERT(stacktop > parent->xcp.sregs[REG_SP]);
stackutil = stacktop - parent->xcp.sregs[REG_SP];
/* Copy goes to child's user stack top */
newtop = (uintptr_t)child->cmn.stack_base_ptr + child->cmn.adj_stack_size;
newsp = newtop - stackutil;
memcpy((void *)newsp, (const void *)parent->xcp.regs[REG_SP], stackutil);
memcpy((void *)newsp, (const void *)parent->xcp.sregs[REG_SP], stackutil);
#ifdef CONFIG_SCHED_THREAD_LOCAL
/* Save child's thread pointer */
@ -169,7 +164,7 @@ pid_t riscv_fork(const struct fork_s *context)
/* Copy the parent integer context (overwrites child's SP and TP) */
memcpy(child->cmn.xcp.regs, parent->xcp.regs, XCPTCONTEXT_SIZE);
memcpy(child->cmn.xcp.regs, parent->xcp.sregs, XCPTCONTEXT_SIZE);
/* Save FPU */
@ -183,8 +178,6 @@ pid_t riscv_fork(const struct fork_s *context)
child->cmn.xcp.regs[REG_TP] = tp;
#endif
up_irq_restore(flags);
/* And, finally, start the child task. On a failure, nxtask_start_fork()
* will discard the TCB by calling nxtask_abort_fork().
*/

2
arch/risc-v/src/common/riscv_swint.c
View File

@ -161,7 +161,7 @@ uintptr_t dispatch_syscall(unsigned int nbr, uintptr_t parm1,
/* Set the user register context to TCB */
rtcb->xcp.regs = context;
rtcb->xcp.sregs = context;
/* Indicate that we are in a syscall handler */