rtld: fix SysV hash function overflow

Quoting from https://maskray.me/blog/2023-04-12-elf-hash-function:

The System V Application Binary Interface (generic ABI) specifies the
ELF object file format. When producing an output executable or shared
object needing a dynamic symbol table (.dynsym), a linker generates a
.hash section with type SHT_HASH to hold a symbol hash table. A DT_HASH
tag is produced to hold the address of .hash.

The function is supposed to return a value no larger than 0x0fffffff.
Unfortunately, there is a bug. When unsigned long consists of more than
32 bits, the return value may be larger than UINT32_MAX. For instance,
elf_hash((const unsigned char *)"\xff\x0f\x0f\x0f\x0f\x0f\x12") returns
0x100000002, which is clearly unintended, as the function should behave
the same way regardless of whether long represents a 32-bit integer or
a 64-bit integer.

Reviewed by:	kib, Fangrui Song
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39517

libexec/rtld-elf/rtld.c

@@ -1796,23 +1796,20 @@ donelist_check(DoneList *dlp, const Obj_Entry *obj)
 }
 
 /*
- * Hash function for symbol table lookup.  Don't even think about changing
+ * SysV hash function for symbol table lookup.  It is a slightly optimized
- * this.  It is specified by the System V ABI.
+ * version of the hash specified by the System V ABI.
  */
-unsigned long
+Elf32_Word
 elf_hash(const char *name)
 {
 	const unsigned char *p = (const unsigned char *)name;
-	unsigned long h = 0;
+	Elf32_Word h = 0;
-	unsigned long g;
 
 	while (*p != '\0') {
 		h = (h << 4) + *p++;
-		if ((g = h & 0xf0000000) != 0)
+		h ^= (h >> 24) & 0xf0;
-			h ^= g >> 24;
-		h &= ~g;
 	}
-	return (h);
+	return (h & 0x0fffffff);
 }
 
 /*

libexec/rtld-elf/rtld.h

@@ -379,7 +379,7 @@ void dump_Elf_Rela(Obj_Entry *, const Elf_Rela *, u_long);
  */
 uintptr_t rtld_round_page(uintptr_t);
 uintptr_t rtld_trunc_page(uintptr_t);
-unsigned long elf_hash(const char *);
+Elf32_Word elf_hash(const char *);
 const Elf_Sym *find_symdef(unsigned long, const Obj_Entry *,
   const Obj_Entry **, int, SymCache *, struct Struct_RtldLockState *);
 void lockdflt_init(void);