[PATCH 3/4] hurd: Fix sc_i386_thread_state layout

[Sergey Bugaev]

The real i386_thread_state Mach structure has an alignment of 8 on
x86_64. However, in struct sigcontext, the compiler was packing sc_gs
(which is the first member of sc_i386_thread_state) into the same 8-byte
slot as sc_error; this resulted in the rest of sc_i386_thread_state
members having wrong offsets relative to each other, and the overall
sc_i386_thread_state layout mismatching that of i386_thread_state.

Fix this by explicitly adding the required padding members, and
statically asserting that this results in the desired alignment.

The same goes for sc_i386_float_state.

Checked on x86_64-gnu.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
---
 sysdeps/mach/hurd/x86/trampoline.c         | 6 ++++++
 sysdeps/mach/hurd/x86_64/bits/sigcontext.h | 8 ++++++++
 2 files changed, 14 insertions(+)

diff --git a/sysdeps/mach/hurd/x86/trampoline.c 
b/sysdeps/mach/hurd/x86/trampoline.c
index 1f92064e..6318c952 100644
--- a/sysdeps/mach/hurd/x86/trampoline.c
+++ b/sysdeps/mach/hurd/x86/trampoline.c
@@ -242,11 +242,17 @@ _hurd_setup_sighandler (struct hurd_sigstate *ss, const 
struct sigaction *action
 
       /* struct sigcontext is laid out so that starting at sc_gs mimics a
         struct i386_thread_state.  */
+      _Static_assert (offsetof (struct sigcontext, sc_i386_thread_state)
+                     % __alignof__ (struct i386_thread_state) == 0,
+                     "sc_i386_thread_state layout mismatch");
       memcpy (&scp->sc_i386_thread_state,
              &state->basic, sizeof (state->basic));
 
       /* struct sigcontext is laid out so that starting at sc_fpkind mimics
         a struct i386_float_state.  */
+      _Static_assert (offsetof (struct sigcontext, sc_i386_float_state)
+                     % __alignof__ (struct i386_float_state) == 0,
+                     "sc_i386_float_state layout mismatch");
       ok = machine_get_state (ss->thread, state, i386_FLOAT_STATE,
                              &state->fpu, &scp->sc_i386_float_state,
                              sizeof (state->fpu));
diff --git a/sysdeps/mach/hurd/x86_64/bits/sigcontext.h 
b/sysdeps/mach/hurd/x86_64/bits/sigcontext.h
index 3a3b34bc..63960544 100644
--- a/sysdeps/mach/hurd/x86_64/bits/sigcontext.h
+++ b/sysdeps/mach/hurd/x86_64/bits/sigcontext.h
@@ -46,6 +46,11 @@ struct sigcontext
     /* Error code associated with this signal (interpreted as `error_t').  */
     int sc_error;
 
+    /* Make sure the below members are properly aligned, and not packed
+       together with sc_error -- otherwise the layout won't match that of
+       i386_thread_state.  */
+    int sc_pad1;
+
     /* All following members are machine-dependent.  The rest of this
        structure is written to be laid out identically to:
        {
@@ -86,6 +91,9 @@ struct sigcontext
     long sc_ursp;              /* This stack pointer is used.  */
     int sc_ss;                 /* Stack segment register.  */
 
+    /* Make sure the below has the same layout as i386_float_state.  */
+    int sc_pad2;
+
     /* Following mimics struct i386_float_state.  Structures and symbolic
        values can be found in <mach/i386/fp_reg.h>.  */
 #define sc_i386_float_state sc_fpkind
-- 
2.40.1