On Sun, Feb 21, 2021 at 12:25:53AM +0000, Sergei Trofimovich wrote:
> In https://bugs.gentoo.org/769614 Dmitry noticed that
> `ptrace(PTRACE_GET_SYSCALL_INFO)` does not work for syscalls called
> via glibc's syscall() wrapper.
>
> ia64 has two ways to call syscalls from userspace: via `break` and via
> `eps` instructions.
>
> The difference is in stack layout:
>
> 1. `eps` creates simple stack frame: no locals, in{0..7} == out{0..8}
> 2. `break` uses userspace stack frame: may be locals (glibc provides
> one), in{0..7} == out{0..8}.
>
> Both work fine in syscall handling cde itself.
>
> But `ptrace(PTRACE_GET_SYSCALL_INFO)` uses unwind mechanism to
> re-extract syscall arguments but it does not account for locals.
>
> The change always skips locals registers. It should not change `eps`
> path as kernel's handler already enforces locals=0 and fixes `break`.
>
> Tested on v5.10 on rx3600 machine (ia64 9040 CPU).
>
> CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> CC: linux-ia64@xxxxxxxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Reported-by: Dmitry V. Levin <ldv@xxxxxxxxxxxx>
> Bug: https://bugs.gentoo.org/769614
> Signed-off-by: Sergei Trofimovich <slyfox@xxxxxxxxxx>
> ---
> arch/ia64/kernel/ptrace.c | 24 ++++++++++++++++++------
> 1 file changed, 18 insertions(+), 6 deletions(-)
>
> diff --git a/arch/ia64/kernel/ptrace.c b/arch/ia64/kernel/ptrace.c
> index c3490ee2daa5..e14f5653393a 100644
> --- a/arch/ia64/kernel/ptrace.c
> +++ b/arch/ia64/kernel/ptrace.c
> @@ -2013,27 +2013,39 @@ static void syscall_get_set_args_cb(struct unw_frame_info *info, void *data)
> {
> struct syscall_get_set_args *args = data;
> struct pt_regs *pt = args->regs;
> - unsigned long *krbs, cfm, ndirty;
> + unsigned long *krbs, cfm, ndirty, nlocals, nouts;
> int i, count;
>
> if (unw_unwind_to_user(info) < 0)
> return;
>
> + /*
> + * We get here via a few paths:
> + * - break instruction: cfm is shared with caller.
> + * syscall args are in out= regs, locals are non-empty.
> + * - epsinstruction: cfm is set by br.call
> + * locals don't exist.
typo: epsinstruction
> + *
> + * For both cases argguments are reachable in cfm.sof - cfm.sol.
typo: argguments
> + * CFM: [ ... | sor: 17..14 | sol : 13..7 | sof : 6..0 ]
> + */
> cfm = pt->cr_ifs;
> + nlocals = (cfm >> 7) & 0x7f; /* aka sol */
> + nouts = (cfm & 0x7f) - nlocals; /* aka sof - sol */
> krbs = (unsigned long *)info->task + IA64_RBS_OFFSET/8;
> ndirty = ia64_rse_num_regs(krbs, krbs + (pt->loadrs >> 19));
>
> count = 0;
> if (in_syscall(pt))
> - count = min_t(int, args->n, cfm & 0x7f);
> + count = min_t(int, args->n, nouts);
>
> + /* Iterate over outs. */
> for (i = 0; i < count; i++) {
> + int j = ndirty + nlocals + i + args->i;
> if (args->rw)
> - *ia64_rse_skip_regs(krbs, ndirty + i + args->i) =
> - args->args[i];
> + *ia64_rse_skip_regs(krbs, j) = args->args[i];
> else
> - args->args[i] = *ia64_rse_skip_regs(krbs,
> - ndirty + i + args->i);
> + args->args[i] = *ia64_rse_skip_regs(krbs, j);
> }
>
> if (!args->rw) {
This stuff is too ia64 specific, so I cannot properly review this patch,
but it definitely fixes ia64 PTRACE_GET_SYSCALL_INFO on entering syscall.
--
ldv
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