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    SubjectRe: [PATCH 1/2] signal: Introduce COMPAT_SIGMINSTKSZ for use in compat_sys_sigaltstack
    On Wed, Jul 25, 2018 at 04:54:27PM +0100, Dave Martin wrote:
    > On Wed, Jul 25, 2018 at 02:45:11PM +0100, Will Deacon wrote:
    > > @@ -3476,7 +3478,8 @@ int restore_altstack(const stack_t __user *uss)
    > > stack_t new;
    > > if (copy_from_user(&new, uss, sizeof(stack_t)))
    > > return -EFAULT;
    > > - (void)do_sigaltstack(&new, NULL, current_user_stack_pointer());
    > > + (void)do_sigaltstack(&new, NULL, current_user_stack_pointer(),
    > > + MINSIGSTKSZ);
    >
    > Why can't this fail?
    >
    > If this fails here we silently go wrong, but...
    >
    > > /* squash all but EFAULT for now */
    > > return 0;
    > > }
    > > @@ -3510,7 +3513,8 @@ static int do_compat_sigaltstack(const compat_stack_t __user *uss_ptr,
    > > uss.ss_size = uss32.ss_size;
    > > }
    > > ret = do_sigaltstack(uss_ptr ? &uss : NULL, &uoss,
    > > - compat_user_stack_pointer());
    > > + compat_user_stack_pointer(),
    > > + COMPAT_MINSIGSTKSZ);
    >
    > If this fails on arm64, we seem to SEGV (see compat_sys_rt_sigreturn()).
    >
    > This patch doesn't introduce this inconsistency, this might be a good
    > opportunity to clean it up.

    I don't think there's an inconsistency here -- both restore_altstack and
    compat_restore_altstack suppress all non--EFAULT errors (remember that uoss
    is NULL in both cases, so the copy_from_user() immediately before the
    do_sigaltstack() call for the native path is all we care about). I think the
    behaviour is: on a sigreturn, if you set the altstack to be an unmapped
    address then you get a SEGV, otherwise if you make it invalid in some other
    way (e.g. too small) then it's ignored and the old altstack remains intact.

    Will

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