Hello,

This patch fixes an internal error in GAS when defining a section using
a symbol that has already been named but not defined. For a minimal
reproducer, try the following input:

a=b
.sect a

The problem is that obj_elf_change_section() happily reuses the symbol
"a" created by equals() without clearing the sy_value field: prior to
this patch, it just set bsym. This caused a problem when attempting to
resolve the section symbol, since resolve_symbol_value() ended up
resolving the symbol as if it were the original symbol created by
equals(), which ends up leaving the section symbol in the undefined
section instead of in section a, hence the call to abort() in
S_SET_SEGMENT().

Testing:
 * New test fails before and passes after the change.
 * Regression tested on a wide variety of targets.

OK for master?

Thanks,
Alex

---

gas/ChangeLog:

2020-07-22  Alex Coplan  <alex.coplan@arm.com>

	* config/obj-elf.c (obj_elf_change_section): When
	repurposing an existing symbol, ensure that we set sy_value as
	per other (fresh) section symbols.
	* testsuite/gas/aarch64/section-symbol-redef.d: New test.
	* testsuite/gas/aarch64/section-symbol-redef.s: Input for test.