Refuse to load non-EXEC/non-RISC-V/non-V1 ELFs (#388)

Stricter validation of ELF binaries improves usability with informative
assertions. This prevents users from loading ELF relocatable files and
binaries compiled for their (non-RISC-V) workstations, for example.

Without this patch, spike would attempt to load nearly any ELF given,
but it would usually fail with an error about debug module accesses,
since the given ELF causes accesses in the debug module's memory space.
Even if spike successfully loaded the ELF file, it would still misbehave
during simulation, for example in the case of ELF relocatable files.

ELF magic numbers come from official ELF documents:
TIS ELF spec v1.2, via Linux Foundation Referenced Specifications
See: https://refspecs.linuxbase.org/

RISC-V magic number comes from official RISC-V ELF documents:
See: riscv/riscv-elf-psabi-doc@60c25981b62c0b43d16142f8a12c8b1e98e60d4d

fesvr/elf.h

@@ -5,6 +5,10 @@
 
 #include <stdint.h>
 
+#define ET_EXEC 2
+#define EM_RISCV 243
+#define EV_CURRENT 1
+
 #define IS_ELF(hdr) \
   ((hdr).e_ident[0] == 0x7f && (hdr).e_ident[1] == 'E' && \
    (hdr).e_ident[2] == 'L'  && (hdr).e_ident[3] == 'F')
@@ -13,6 +17,9 @@
 #define IS_ELF64(hdr) (IS_ELF(hdr) && (hdr).e_ident[4] == 2)
 #define IS_ELFLE(hdr) (IS_ELF(hdr) && (hdr).e_ident[5] == 1)
 #define IS_ELFBE(hdr) (IS_ELF(hdr) && (hdr).e_ident[5] == 2)
+#define IS_ELF_EXEC(hdr) (IS_ELF(hdr) && (hdr).e_type == ET_EXEC)
+#define IS_ELF_RISCV(hdr) (IS_ELF(hdr) && (hdr).e_machine == EM_RISCV)
+#define IS_ELF_VCURRENT(hdr) (IS_ELF(hdr) && (hdr).e_version == EV_CURRENT)
 
 #define PT_LOAD 1
 

fesvr/elfloader.cc

@@ -32,6 +32,9 @@ std::map<std::string, uint64_t> load_elf(const char* fn, memif_t* memif, reg_t*
   const Elf64_Ehdr* eh64 = (const Elf64_Ehdr*)buf;
   assert(IS_ELF32(*eh64) || IS_ELF64(*eh64));
   assert(IS_ELFLE(*eh64));
+  assert(IS_ELF_EXEC(*eh64));
+  assert(IS_ELF_RISCV(*eh64));
+  assert(IS_ELF_VCURRENT(*eh64));
 
   std::vector<uint8_t> zeros;
   std::map<std::string, uint64_t> symbols;