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powerpc: Use trap metadata to prevent double restart rather than zeroing trap

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It's not very nice to zero trap for this, because then system calls no
longer have trap_is_syscall(regs) invariant, and we can't distinguish
between sc and scv system calls (in a later patch).

Take one last unused bit from the low bits of the pt_regs.trap word
for this instead. There is not a really good reason why it should be
in trap as opposed to another field, but trap has some concept of
flags and it exists. Ideally I think we would move trap to 2-byte
field and have 2 more bytes available independently.

Add a selftests case for this, which can be seen to fail if
trap_norestart() is changed to return false.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Make them static inlines]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200507121332.2233629-4-mpe@ellerman.id.au
This commit is contained in:
Nicholas Piggin
2020-05-07 22:13:32 +10:00
committed by Michael Ellerman
parent 912237ea16
commit 4e0e45b07d
6 changed files with 201 additions and 16 deletions
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10
arch/powerpc/kernel/signal_64.c
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@@ -350,8 +350,8 @@ static long restore_sigcontext(struct task_struct *tsk, sigset_t *set, int sig,
err |= __get_user(regs->link, &sc->gp_regs[PT_LNK]);
err |= __get_user(regs->xer, &sc->gp_regs[PT_XER]);
err |= __get_user(regs->ccr, &sc->gp_regs[PT_CCR]);
/* skip SOFTE */
regs->trap = 0;
/* Don't allow userspace to set SOFTE */
set_trap_norestart(regs);
err |= __get_user(regs->dar, &sc->gp_regs[PT_DAR]);
err |= __get_user(regs->dsisr, &sc->gp_regs[PT_DSISR]);
err |= __get_user(regs->result, &sc->gp_regs[PT_RESULT]);
@@ -472,10 +472,8 @@ static long restore_tm_sigcontexts(struct task_struct *tsk,
&sc->gp_regs[PT_XER]);
err |= __get_user(tsk->thread.ckpt_regs.ccr,
&sc->gp_regs[PT_CCR]);
/* Don't allow userspace to set the trap value */
regs->trap = 0;
/* Don't allow userspace to set SOFTE */
set_trap_norestart(regs);
/* These regs are not checkpointed; they can go in 'regs'. */
err |= __get_user(regs->dar, &sc->gp_regs[PT_DAR]);
err |= __get_user(regs->dsisr, &sc->gp_regs[PT_DSISR]);