Re: [PATCH, binutils, ARM 7/11] Support for dedicated stub section with padding

[Thomas Preudhomme <thomas dot preudhomme at foss dot arm dot com>]

On Tuesday 29 March 2016 15:40:17 Thomas Preudhomme wrote:
> Hi,
> 
> This patch is part of a patch series to add support for ARMv8-M Security
> Extensions[1] to GNU ld. This specific patch adds support for padding the
> dedicated section of a given veneer type.
> 
> ARM v8-M Security Extensions require [3] secure gateway veneers to be
> generated for (secure) entry function in order for code to transition from
> non-secure state to secure state when calling these entry function. One
> requirement for these veneers [4] is that a consecutive sequence of them
> must be padded to 32 bytes to make sure that no bit pattern of the SG
> instruction is found in non secure callable memory. This patch adds support
> for padding a dedicated section used for a given type of veneers, thus
> respecting this requirement.
> 
> 
> [1] Software requirements for ARMv8-M security extension are described in
> document ARM-ECM-0359818 [2]
> [2] Available on http://infocenter.arm.com in Developer guides and articles
> > Software development > ARMÂv8-M Security Extensions: Requirements on
> Development Tools
> [3] See section 3.4.3 and requirement 44 of ARM-ECM-0359818 [2]
> [4] requirement 12 and 13 and following comment of ARM-ECM-0359818 [2]
> 

Please find an updated version without the switch case for the 
arm_dedicated_stub_* functions.

ChangeLog entry is unchanged:

*** bfd/ChangeLog ***

2016-02-17  Thomas Preud'homme  <thomas.preudhomme@arm.com>

        * elf32-arm.c (arm_dedicated_stub_section_padding): New function.
        (elf32_arm_size_stubs): Declare stub_type in a more outer scope and
        account for padding for stub section requiring one.
        (elf32_arm_build_stubs): Add comment to stress the importance of
        zeroing veneer section content.


diff --git a/bfd/elf32-arm.c b/bfd/elf32-arm.c
index 
ab878b20905c907b3496317c7ed1c9c698d850c9..f2b50667ef50572488e4d868ce40e67580356343 
100644
--- a/bfd/elf32-arm.c
+++ b/bfd/elf32-arm.c
@@ -4435,6 +4435,18 @@ arm_stub_sym_claimed (enum elf32_arm_stub_type 
stub_type)
   return FALSE;
 }
 
+/* Returns the padding needed for the dedicated section used stubs of type
+   STUB_TYPE.  */
+
+static int
+arm_dedicated_stub_section_padding (enum elf32_arm_stub_type stub_type)
+{
+  if (stub_type >= max_stub_type)
+    abort ();  /* Should be unreachable.  */
+
+  return 0;
+}
+
 static bfd_boolean
 arm_build_one_stub (struct bfd_hash_entry *gen_entry,
 		    void * in_arg)
@@ -5411,6 +5423,7 @@ elf32_arm_size_stubs (bfd *output_bfd,
       bfd *input_bfd;
       unsigned int bfd_indx;
       asection *stub_sec;
+      enum elf32_arm_stub_type stub_type;
       bfd_boolean stub_changed = FALSE;
       unsigned prev_num_a8_fixes = num_a8_fixes;
 
@@ -5466,7 +5479,6 @@ elf32_arm_size_stubs (bfd *output_bfd,
 	      for (; irela < irelaend; irela++)
 		{
 		  unsigned int r_type, r_indx;
-		  enum elf32_arm_stub_type stub_type;
 		  asection *sym_sec;
 		  bfd_vma sym_value;
 		  bfd_vma destination;
@@ -5765,7 +5777,27 @@ elf32_arm_size_stubs (bfd *output_bfd,
 	  stub_sec->size = 0;
 	}
 
+      /* Compute stub section size, considering padding.  */
       bfd_hash_traverse (&htab->stub_hash_table, arm_size_one_stub, htab);
+      for (stub_type = arm_stub_none + 1; stub_type < max_stub_type;
+	   stub_type++)
+	{
+	  int size, padding;
+	  asection **stub_sec_p;
+
+	  padding = arm_dedicated_stub_section_padding (stub_type);
+	  stub_sec_p = arm_dedicated_stub_input_section_ptr (htab, stub_type);
+	  /* Skip if no stub input section or no stub section padding
+	     required.  */
+	  if ((stub_sec_p != NULL && *stub_sec_p == NULL) || padding == 0)
+	    continue;
+	  /* Stub section padding required but no dedicated section.  */
+	  BFD_ASSERT (stub_sec_p);
+
+	  size = (*stub_sec_p)->size;
+	  size = (size + padding - 1) & ~(padding - 1);
+	  (*stub_sec_p)->size = size;
+	}
 
       /* Add Cortex-A8 erratum veneers to stub section sizes too.  */
       if (htab->fix_cortex_a8)
@@ -5873,7 +5905,8 @@ elf32_arm_build_stubs (struct bfd_link_info *info)
       if (!strstr (stub_sec->name, STUB_SUFFIX))
 	continue;
 
-      /* Allocate memory to hold the linker stubs.  */
+      /* Allocate memory to hold the linker stubs.  Zeroing the stub sections
+	 must at least be done for stub section requiring padding.  */
       size = stub_sec->size;
       stub_sec->contents = (unsigned char *) bfd_zalloc (htab->stub_bfd, 
size);
       if (stub_sec->contents == NULL && size != 0)


Best regards,

Thomas