linux-user/hexagon: fix signal context save & restore

This patch fixes the issue originally reported in
this thread:

https://lists.gnu.org/archive/html/qemu-devel/2021-11/msg01102.html

The root cause of the issue is a bug in the hexagon specific
logic for saving & restoring context during signal delivery.
The CPU state has two different representations for the
predicate registers. The current logic saves & restores only
the aliased HEX_REG_P3_O register, which is part of env->gpr[]
field in the CPU state, but not the individual byte-level
predicate registers (pO, p1, p2, p3) backed by env->pred[].

Since all predicated instructions refer only to the
indiviual registers, switching to and back from a signal handler
can clobber these registers if the signal handler writes to them
causing the normal application code to behave unpredictably when
context is restored.

In the reported issue with the 'signals' test, since the updated
hexagon toolchain had built musl with -O2, the functions called
from non_trivial_free were inlined. This meant that the code
emitted reused predicate P0 computed in the entry translation
block of the function non_trivial_free in one of the child TB
as part of an assertion. Since P0 is clobbered by the signal
handler in the signals test, the assertion in non_trivial_free
fails incorectly. Since musl for hexagon implements the 'abort'
function by deliberately writing to memory via null pointer,
this causes the test to fail with segmentation fault.

This patch modifies the signal context save & restore logic
to include the individual p0, p1, p2, p3 and excludes the
32b p3_0 register since its value is derived from the former
registers. It also adds a new test case that reliabily
reproduces the issue for all four predicate registers.

Buglink: https://github.com/quic/toolchain_for_hexagon/issues/6
Signed-off-by: Mukilan Thiyagarajan <quic_mthiyaga@quicinc.com>
Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Reviewed-by: Taylor Simpson <tsimpson@quicinc.com>
Message-Id: <20221229092006.10709-2-quic_mthiyaga@quicinc.com>

1 files changed, 11 insertions, 6 deletions

diff --git a/linux-user/hexagon/signal.c b/linux-user/hexagon/signal.c
index ad4e3822d5..60fa7e1bce 100644
--- a/linux-user/hexagon/signal.c
+++ b/linux-user/hexagon/signal.c
@@ -39,15 +39,12 @@ struct target_sigcontext {
     target_ulong m0;
     target_ulong m1;
     target_ulong usr;
-    target_ulong p3_0;
     target_ulong gp;
     target_ulong ugp;
     target_ulong pc;
     target_ulong cause;
     target_ulong badva;
-    target_ulong pad1;
-    target_ulong pad2;
-    target_ulong pad3;
+    target_ulong pred[NUM_PREGS];
 };
 
 struct target_ucontext {
@@ -118,10 +115,14 @@ static void setup_sigcontext(struct target_sigcontext *sc, CPUHexagonState *env)
     __put_user(env->gpr[HEX_REG_M0], &sc->m0);
     __put_user(env->gpr[HEX_REG_M1], &sc->m1);
     __put_user(env->gpr[HEX_REG_USR], &sc->usr);
-    __put_user(env->gpr[HEX_REG_P3_0], &sc->p3_0);
     __put_user(env->gpr[HEX_REG_GP], &sc->gp);
     __put_user(env->gpr[HEX_REG_UGP], &sc->ugp);
     __put_user(env->gpr[HEX_REG_PC], &sc->pc);
+
+    int i;
+    for (i = 0; i < NUM_PREGS; i++) {
+        __put_user(env->pred[i], &(sc->pred[i]));
+    }
 }
 
 static void setup_ucontext(struct target_ucontext *uc,
@@ -230,10 +231,14 @@ static void restore_sigcontext(CPUHexagonState *env,
     __get_user(env->gpr[HEX_REG_M0], &sc->m0);
     __get_user(env->gpr[HEX_REG_M1], &sc->m1);
     __get_user(env->gpr[HEX_REG_USR], &sc->usr);
-    __get_user(env->gpr[HEX_REG_P3_0], &sc->p3_0);
     __get_user(env->gpr[HEX_REG_GP], &sc->gp);
     __get_user(env->gpr[HEX_REG_UGP], &sc->ugp);
     __get_user(env->gpr[HEX_REG_PC], &sc->pc);
+
+    int i;
+    for (i = 0; i < NUM_PREGS; i++) {
+        __get_user(env->pred[i], &(sc->pred[i]));
+    }
 }
 
 static void restore_ucontext(CPUHexagonState *env, struct target_ucontext *uc)