arm: ubsan: shift exponent 4G

* reloc.c (N_ONES): Handle N=0.
	* elf32-arm.c (elf32_arm_howto_table_1): Set complain_overflow_dont
	for R_ARM_TLS_DESCSEQ and R_ARM_THM_TLS_DESCSEQ.

bfd/ChangeLog

@@ -1,3 +1,9 @@
+2020-09-01  Alan Modra  <amodra@gmail.com>
+
+	* reloc.c (N_ONES): Handle N=0.
+	* elf32-arm.c (elf32_arm_howto_table_1): Set complain_overflow_dont
+	for R_ARM_TLS_DESCSEQ and R_ARM_THM_TLS_DESCSEQ.
+
 2020-09-01  Alan Modra  <amodra@gmail.com>
 
 	PR 26429

bfd/elf32-arm.c

@@ -1391,7 +1391,7 @@ static reloc_howto_type elf32_arm_howto_table_1[] =
 	 0,			/* bitsize */
 	 FALSE,			/* pc_relative */
 	 0,			/* bitpos */
-	 complain_overflow_bitfield,/* complain_on_overflow */
+	 complain_overflow_dont,/* complain_on_overflow */
 	 bfd_elf_generic_reloc,	/* special_function */
 	 "R_ARM_TLS_DESCSEQ",	/* name */
 	 FALSE,			/* partial_inplace */
@@ -1683,7 +1683,7 @@ static reloc_howto_type elf32_arm_howto_table_1[] =
 	 0,			/* bitsize */
 	 FALSE,			/* pc_relative */
 	 0,			/* bitpos */
-	 complain_overflow_bitfield,/* complain_on_overflow */
+	 complain_overflow_dont,/* complain_on_overflow */
 	 bfd_elf_generic_reloc,	/* special_function */
 	 "R_ARM_THM_TLS_DESCSEQ",/* name */
 	 FALSE,			/* partial_inplace */

bfd/reloc.c

@@ -432,8 +432,9 @@ DESCRIPTION
 .
 */
 
-/* N_ONES produces N one bits, without overflowing machine arithmetic.  */
-#define N_ONES(n) (((((bfd_vma) 1 << ((n) - 1)) - 1) << 1) | 1)
+/* N_ONES produces N one bits, without undefined behaviour for N
+   between zero and the number of bits in a bfd_vma.  */
+#define N_ONES(n) ((n) == 0 ? 0 : ((bfd_vma) 1 << ((n) - 1) << 1) - 1)
 
 /*
 FUNCTION