[glibc] hurd: Fix sc_i386_thread_state layout

[glibc] hurd: Fix sc_i386_thread_state layout

[Samuel Thibault]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=e333759f7752593a69a8f9920a247ed3878fafef

commit e333759f7752593a69a8f9920a247ed3878fafef
Author: Sergey Bugaev <bugaevc@gmail.com>
Date:   Mon May 15 11:33:22 2023 +0300

    hurd: Fix sc_i386_thread_state layout
    
    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>
    Message-Id: <20230515083323.1358039-4-bugaevc@gmail.com>

Diff:
---
 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 1f92064ebf..6318c9528a 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 3a3b34bc0a..6396054463 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