[SFrame-V3] gas: sframe: allow SFrame FDEs with 0 FREs for signal frames

Update the SFrame generation logic in GAS to emit Function Descriptor Entries
(FDEs) for signal frames even when no Frame Row Entries (FREs) could be
generated.

Previously, create_sframe_all () would discard any FDE that failed
translation or resulted in zero FREs. However, for signal frames (marked
with .cfi_signal_frame), preserving the FDE may be valuable even without
stack offsets. The presence of the SFrame FDE with the 'Signal'
attribute may allow stack tracers to identify the frame as a signal
trampoline and potentially apply fallback handling, rather than treating
the PC range as having no stack trace info at all.

The patch modifies create_sframe_all () to detect translation errors for
signal frames, effectively allowing the generation of an empty FDE (0 FREs)
marked with the 'S' attribute.

TBD:
 - But the warnings are already issued, we may emit SFrame FDE with no
   FREs.  Somewhat misleading.

gas/
	* gen-sframe.c (sframe_fde_free): Add NULL check for safety.
	(create_sframe_all): Allow FDEs for signal frames even if translation
	encountered errors or produced no FREs.
gas/testsuite/
	* gas/cfi-sframe/cfi-sframe.exp: New test.
	* gas/cfi-sframe/cfi-sframe-x86_64-signal-1.d: New test ensuring
	signal frame FDE is emitted with 0 FREs.
	* gas/cfi-sframe/cfi-sframe-x86_64-signal-1.s: New test.

diff --git a/gas/gen-sframe.c b/gas/gen-sframe.c
index 7a92c0d26a10cdb457875fafc773ed864cf888a1..21994590f3f7b61a94c70ef6f842e9960ac033b8 100644 (file)
--- a/gas/gen-sframe.c
+++ b/gas/gen-sframe.c
@@ -573,7 +573,12 @@ sframe_fde_alloc (void)
 static void
 sframe_fde_free (struct sframe_func_entry *sframe_fde)
 {
-  sframe_row_entry_free (sframe_fde->sframe_fres);
+  if (sframe_fde == NULL)
+    return;
+
+  if (sframe_fde->sframe_fres)
+    sframe_row_entry_free (sframe_fde->sframe_fres);
+
   XDELETE (sframe_fde);
 }
 
@@ -2100,12 +2105,15 @@ create_sframe_all (void)
       /* Initialize the translation context with information anew.  */
       sframe_xlate_ctx_init (xlate_ctx);
 
-      /* Process and link SFrame FDEs if no error.  Also skip adding an SFrame
-        FDE if it does not contain any SFrame FREs.  There is little use of an
-        SFrame FDE if there is no stack tracing information for the
-        function.  */
+      /* Process and link SFrame FDEs if no error.  */
       int err = sframe_do_fde (xlate_ctx, dw_fde);
-      if (err || xlate_ctx->num_xlate_fres == 0)
+      if (err && get_dw_fde_signal_p (dw_fde))
+       {
+         sframe_xlate_ctx_cleanup (xlate_ctx);
+         err = SFRAME_XLATE_OK;
+       }
+
+      if (err)
        {
          sframe_xlate_ctx_cleanup (xlate_ctx);
          sframe_fde_free (sframe_fde);

diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-signal-1.d b/gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-signal-1.d
new file mode 100644 (file)
index 0000000..adffa7d
--- /dev/null
+++ b/gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-signal-1.d
@@ -0,0 +1,18 @@
+#as: --gsframe
+#warning: \.cfi\_escape with op \(0xf\)
+#objdump: --sframe=.sframe
+#name: Signal Frame with no FREs
+#...
+Contents of the SFrame section .sframe:
+  Header :
+
+    Version: SFRAME_VERSION_3
+    Flags: SFRAME_F_FDE_FUNC_START_PCREL
+    CFA fixed RA offset: -8
+    Num FDEs: 1
+    Num FREs: 0
+
+  Function Index :
+
+    func idx \[0\]: pc = 0x0, size = 3 bytes, attr = "S"
+    STARTPC + CFA + FP + RA +

diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-signal-1.s b/gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-signal-1.s
new file mode 100644 (file)
index 0000000..2dc6e91
--- /dev/null
+++ b/gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-signal-1.s
@@ -0,0 +1,12 @@
+       .type   foo, @function
+foo:
+       .cfi_startproc
+       .cfi_signal_frame
+# DW_CFA_def_cfa_expression, length, DW_OP_breg9, (sleb)offset, DW_OP_deref
+        .cfi_escape 0xf,0x3,0x79,0x78,0x6
+       nop
+       pop     %rbp
+       .cfi_offset 6, -16
+       ret
+       .cfi_endproc
+       .size   foo, .-foo

diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe.exp b/gas/testsuite/gas/cfi-sframe/cfi-sframe.exp
index a352b325fd6ae372c64d24bafe97a8c7af32474f..f86150574c90521c94835656cf43204e18d04342 100644 (file)
--- a/gas/testsuite/gas/cfi-sframe/cfi-sframe.exp
+++ b/gas/testsuite/gas/cfi-sframe/cfi-sframe.exp
@@ -63,6 +63,7 @@ if { [istarget "x86_64-*-*"] && [gas_sframe_check] } then {
        run_dump_test "cfi-sframe-x86_64-2"
        run_dump_test "cfi-sframe-x86_64-3"
        run_dump_test "cfi-sframe-x86_64-pr33170"
+       run_dump_test "cfi-sframe-x86_64-signal-1"
        run_dump_test "cfi-sframe-x86_64-empty-1"
        run_dump_test "cfi-sframe-x86_64-empty-2"
        run_dump_test "cfi-sframe-x86_64-empty-3"