On Wed, Oct 09, 2019 at 09:26:05AM +0100, Cristian Marussi wrote:
> Introduce a new common utility function get_current_context() which can be
> used to grab a ucontext without the help of libc, and also to detect if
> such ucontext has been successfully used by placing it on the stack as a
> fake sigframe.
Reviewed-by: Dave Martin <Dave.Martin@xxxxxxx>
>
> Signed-off-by: Cristian Marussi <cristian.marussi@xxxxxxx>
> ---
> v7 --> v8
> - added new in v8 (splitted out from v7 05/11)
> ---
> .../selftests/arm64/signal/test_signals.h | 6 +-
> .../arm64/signal/test_signals_utils.c | 31 ++++++
> .../arm64/signal/test_signals_utils.h | 98 +++++++++++++++++++
> 3 files changed, 134 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/arm64/signal/test_signals.h b/tools/testing/selftests/arm64/signal/test_signals.h
> index 901521188202..4fd3ba01e3b1 100644
> --- a/tools/testing/selftests/arm64/signal/test_signals.h
> +++ b/tools/testing/selftests/arm64/signal/test_signals.h
> @@ -88,8 +88,12 @@ struct tdescr {
> /* optional sa_flags for the installed handler */
> int sa_flags;
> ucontext_t saved_uc;
> + /* used by get_current_ctx() */
> + size_t live_sz;
> + ucontext_t *live_uc;
> + volatile sig_atomic_t live_uc_valid;
> /* optional test private data */
> - void *priv;
> + void *priv;
>
> /* a custom setup: called alternatively to default_setup */
> int (*setup)(struct tdescr *td);
> diff --git a/tools/testing/selftests/arm64/signal/test_signals_utils.c b/tools/testing/selftests/arm64/signal/test_signals_utils.c
> index e8bbe36c2660..222148568adf 100644
> --- a/tools/testing/selftests/arm64/signal/test_signals_utils.c
> +++ b/tools/testing/selftests/arm64/signal/test_signals_utils.c
> @@ -11,14 +11,19 @@
> #include <linux/auxvec.h>
> #include <ucontext.h>
>
> +#include <asm/unistd.h>
> +
> #include <kselftest.h>
>
> #include "test_signals.h"
> #include "test_signals_utils.h"
> #include "testcases/testcases.h"
>
> +
> extern struct tdescr *current;
>
> +static int sig_copyctx = SIGTRAP;
> +
> static char const *const feats_names[FMAX_END] = {
> " SSBS ",
> " PAN ",
> @@ -156,6 +161,20 @@ static bool handle_signal_ok(struct tdescr *td,
> return true;
> }
>
> +static bool handle_signal_copyctx(struct tdescr *td,
> + siginfo_t *si, void *uc)
> +{
> + /* Mangling PC to avoid loops on original BRK instr */
> + ((ucontext_t *)uc)->uc_mcontext.pc += 4;
> + memcpy(td->live_uc, uc, td->live_sz);
> + ASSERT_GOOD_CONTEXT(td->live_uc);
> + td->live_uc_valid = 1;
> + fprintf(stderr,
> + "GOOD CONTEXT grabbed from sig_copyctx handler\n");
> +
> + return true;
> +}
> +
> static void default_handler(int signum, siginfo_t *si, void *uc)
> {
> if (current->sig_unsupp && signum == current->sig_unsupp &&
> @@ -167,6 +186,9 @@ static void default_handler(int signum, siginfo_t *si, void *uc)
> } else if (current->sig_ok && signum == current->sig_ok &&
> handle_signal_ok(current, si, uc)) {
> fprintf(stderr, "Handled SIG_OK\n");
> + } else if (signum == sig_copyctx && current->live_uc &&
> + handle_signal_copyctx(current, si, uc)) {
> + fprintf(stderr, "Handled SIG_COPYCTX\n");
> } else {
> if (signum == SIGALRM && current->timeout) {
> fprintf(stderr, "-- Timeout !\n");
> @@ -221,6 +243,15 @@ static inline int default_trigger(struct tdescr *td)
>
> int test_init(struct tdescr *td)
> {
> + if (td->sig_trig == sig_copyctx) {
> + fprintf(stdout,
> + "Signal %d is RESERVED, cannot be used as a trigger. Aborting\n",
> + sig_copyctx);
> + return 0;
> + }
> + /* just in case */
> + unblock_signal(sig_copyctx);
> +
> td->minsigstksz = getauxval(AT_MINSIGSTKSZ);
> if (!td->minsigstksz)
> td->minsigstksz = MINSIGSTKSZ;
> diff --git a/tools/testing/selftests/arm64/signal/test_signals_utils.h b/tools/testing/selftests/arm64/signal/test_signals_utils.h
> index 5e3a2b7aaa8b..fd67b1f23c41 100644
> --- a/tools/testing/selftests/arm64/signal/test_signals_utils.h
> +++ b/tools/testing/selftests/arm64/signal/test_signals_utils.h
> @@ -4,6 +4,10 @@
> #ifndef __TEST_SIGNALS_UTILS_H__
> #define __TEST_SIGNALS_UTILS_H__
>
> +#include <assert.h>
> +#include <stdio.h>
> +#include <string.h>
> +
> #include "test_signals.h"
>
> int test_init(struct tdescr *td);
> @@ -17,4 +21,98 @@ static inline bool feats_ok(struct tdescr *td)
> return (td->feats_required & td->feats_supported) == td->feats_required;
> }
>
> +/*
> + * Obtaining a valid and full-blown ucontext_t from userspace is tricky:
> + * libc getcontext does() not save all the regs and messes with some of
> + * them (pstate value in particular is not reliable).
> + *
> + * Here we use a service signal to grab the ucontext_t from inside a
> + * dedicated signal handler, since there, it is populated by Kernel
> + * itself in setup_sigframe(). The grabbed context is then stored and
> + * made available in td->live_uc.
> + *
> + * As service-signal is used a SIGTRAP induced by a 'brk' instruction,
> + * because here we have to avoid syscalls to trigger the signal since
> + * they would cause any SVE sigframe content (if any) to be removed.
> + *
> + * Anyway this function really serves a dual purpose:
> + *
> + * 1. grab a valid sigcontext into td->live_uc for result analysis: in
> + * such case it returns 1.
> + *
> + * 2. detect if, somehow, a previously grabbed live_uc context has been
> + * used actively with a sigreturn: in such a case the execution would have
> + * magically resumed in the middle of this function itself (seen_already==1):
> + * in such a case return 0, since in fact we have not just simply grabbed
> + * the context.
> + *
> + * This latter case is useful to detect when a fake_sigreturn test-case has
> + * unexpectedly survived without hitting a SEGV.
> + *
> + * Note that the case of runtime dynamically sized sigframes (like in SVE
> + * context) is still NOT addressed: sigframe size is supposed to be fixed
> + * at sizeof(ucontext_t).
> + */
> +static __always_inline bool get_current_context(struct tdescr *td,
> + ucontext_t *dest_uc)
> +{
> + static volatile bool seen_already;
> +
> + assert(td && dest_uc);
> + /* it's a genuine invocation..reinit */
> + seen_already = 0;
> + td->live_uc_valid = 0;
> + td->live_sz = sizeof(*dest_uc);
> + memset(dest_uc, 0x00, td->live_sz);
> + td->live_uc = dest_uc;
> + /*
> + * Grab ucontext_t triggering a SIGTRAP.
> + *
> + * Note that:
> + * - live_uc_valid is declared volatile sig_atomic_t in
> + * struct tdescr since it will be changed inside the
> + * sig_copyctx handler
> + * - the additional 'memory' clobber is there to avoid possible
> + * compiler's assumption on live_uc_valid and the content
> + * pointed by dest_uc, which are all changed inside the signal
> + * handler
> + * - BRK causes a debug exception which is handled by the Kernel
> + * and finally causes the SIGTRAP signal to be delivered to this
> + * test thread. Since such delivery happens on the ret_to_user()
> + * /do_notify_resume() debug exception return-path, we are sure
> + * that the registered SIGTRAP handler has been run to completion
> + * before the execution path is restored here: as a consequence
> + * we can be sure that the volatile sig_atomic_t live_uc_valid
> + * carries a meaningful result. Being in a single thread context
> + * we'll also be sure that any access to memory modified by the
> + * handler (namely ucontext_t) will be visible once returned.
> + * - note that since we are using a breakpoint instruction here
> + * to cause a SIGTRAP, the ucontext_t grabbed from the signal
> + * handler would naturally contain a PC pointing exactly to this
> + * BRK line, which means that, on return from the signal handler,
> + * or if we place the ucontext_t on the stack to fake a sigreturn,
> + * we'll end up in an infinite loop of BRK-SIGTRAP-handler.
> + * For this reason we take care to artificially move forward the
> + * PC to the next instruction while inside the signal handler.
> + */
> + asm volatile ("brk #666"
> + : "+m" (*dest_uc)
> + :
> + : "memory");
> +
> + /*
> + * If we get here with seen_already==1 it implies the td->live_uc
> + * context has been used to get back here....this probably means
> + * a test has failed to cause a SEGV...anyway live_uc does not
> + * point to a just acquired copy of ucontext_t...so return 0
> + */
> + if (seen_already) {
> + fprintf(stdout,
> + "Unexpected successful sigreturn detected: live_uc is stale !\n");
> + return 0;
> + }
> + seen_already = 1;
> +
> + return td->live_uc_valid;
> +}
> #endif
> --
> 2.17.1
>
>
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