RISC-V: Only provide the single-letter extensions in HWCAP

The recent refactoring led to us leaking some HWCAP bits to userspace
that didn't make much sense.  With any luck we'll have a better scheme
soon, but for now just mask off those bits to avoid polluting userspace.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230202233832.11036-1-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/elf.h

@@ -14,6 +14,7 @@
 #include <asm/auxvec.h>
 #include <asm/byteorder.h>
 #include <asm/cacheinfo.h>
+#include <asm/hwcap.h>
 
 /*
  * These are used to set parameters in the core dumps.
@@ -59,12 +60,13 @@ extern bool compat_elf_check_arch(Elf32_Ehdr *hdr);
 #define STACK_RND_MASK		(0x3ffff >> (PAGE_SHIFT - 12))
 #endif
 #endif
+
 /*
- * This yields a mask that user programs can use to figure out what
- * instruction set this CPU supports.  This could be done in user space,
- * but it's not easy, and we've already done it here.
+ * Provides information on the availiable set of ISA extensions to userspace,
+ * via a bitmap that coorespends to each single-letter ISA extension.  This is
+ * essentially defunct, but will remain for compatibility with userspace.
  */
-#define ELF_HWCAP	(elf_hwcap)
+#define ELF_HWCAP	(elf_hwcap & ((1UL << RISCV_ISA_EXT_BASE) - 1))
 extern unsigned long elf_hwcap;
 
 /*

arch/riscv/include/asm/hwcap.h

@@ -52,18 +52,10 @@
 
 #include <linux/jump_label.h>
 
-/*
- * This yields a mask that user programs can use to figure out what
- * instruction set this cpu supports.
- */
-#define ELF_HWCAP		(elf_hwcap)
-
 enum {
 	CAP_HWCAP = 1,
 };
 
-extern unsigned long elf_hwcap;
-
 struct riscv_isa_ext_data {
 	/* Name of the extension displayed to userspace via /proc/cpuinfo */
 	char uprop[RISCV_ISA_EXT_NAME_LEN_MAX];