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[bfd,arm] Don't assert on suspicious build attributes in input file

Message ID 99e452dd-eba7-ebc5-74f2-c17b71872867@arm.com
State New
Headers show

Commit Message

Richard Earnshaw (lists) June 8, 2017, 2:07 p.m. UTC
It's generally a bad idea to use assertions to validate our idea of what
an input file looks like.  We need to be as liberal as possible in what
we accept with respect to standards and conservative with what we produce.

Currently, if gcc is used to produce an assembler file which contains
only data, but the FPU is set to fpv4-sp-d16 and mfloat-abi=hard, then
the following attributes will be set in the output:

        .cpu arm7tdmi
        .eabi_attribute 27, 1   @ Tag_ABI_HardFP_use
        .eabi_attribute 28, 1   @ Tag_ABI_VFP_args
        .eabi_attribute 20, 1   @ Tag_ABI_FP_denormal
        .eabi_attribute 21, 1   @ Tag_ABI_FP_exceptions
        .eabi_attribute 23, 3   @ Tag_ABI_FP_number_model
        .eabi_attribute 24, 1   @ Tag_ABI_align8_needed
        .eabi_attribute 25, 1   @ Tag_ABI_align8_preserved
        .eabi_attribute 26, 2   @ Tag_ABI_enum_size
        .eabi_attribute 30, 6   @ Tag_ABI_optimization_goals
        .eabi_attribute 34, 0   @ Tag_CPU_unaligned_access
        .eabi_attribute 18, 4   @ Tag_ABI_PCS_wchar_t

There is then no .fpu directive to cause Tag_FP_arch to be set, because
there are no functions containing code in the object file.  If this
object file is assembled by hand, but without -mfpu on the invocation of
the assembler, then the build attributes produced will trigger an
assertion during linking.

Thinking about the build attributes, the combination of a
single-precision only implementation of no floating-point architecture
is still no floating-point architecture.  Hence the assertion on the
input BFD in the linker makes no real sense.

We should, however, be more conservative in what we generate, so I've
left the assertion on the output bfd in place; I don't think we can
trigger it with this change since we never merge the problematic tags
from a perversely generated input file.

	* elf32-arm.c (elf32_arm_merge_eabi_attributes): Remove
	assertion that the input bfd has Tag_FP_ARCH non-zero if
	Tag_ABI_HardFP_use is non-zero.  Add clarifying comments.

I'll commit this shortly.

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diff --git a/bfd/elf32-arm.c b/bfd/elf32-arm.c
index 0a78595..85ecca5 100644
--- a/bfd/elf32-arm.c
+++ b/bfd/elf32-arm.c
@@ -13816,6 +13816,9 @@  elf32_arm_merge_eabi_attributes (bfd *ibfd, struct bfd_link_info *info)
 		 follow the requirement of the input.  */
 	      if (out_attr[i].i == 0)
+		  /* This assert is still reasonable, we shouldn't
+		     produce the suspicious build attribute
+		     combination (See below for in_attr).  */
 		  BFD_ASSERT (out_attr[Tag_ABI_HardFP_use].i == 0);
 		  out_attr[i].i = in_attr[i].i;
@@ -13826,7 +13829,13 @@  elf32_arm_merge_eabi_attributes (bfd *ibfd, struct bfd_link_info *info)
 		 nothing.  */
 	      else if (in_attr[i].i == 0)
-		  BFD_ASSERT (in_attr[Tag_ABI_HardFP_use].i == 0);
+		  /* We used to assert that Tag_ABI_HardFP_use was
+		     zero here, but we should never assert when
+		     consuming an object file that has suspicious
+		     build attributes.  The single precision variant
+		     of 'no FP architecture' is still 'no FP
+		     architecture', so we just ignore the tag in this
+		     case.  */