// SPDX-License-Identifier: GPL-2.0
#include <byteswap.h>
#include <elf.h>
#include <endian.h>
#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#ifdef be32toh
/* If libc provides [bl]e{32,64}toh() then we'll use them */
#elif BYTE_ORDER == LITTLE_ENDIAN
# define be32toh(x)     bswap_32(x)
# define le32toh(x)     (x)
# define be64toh(x)     bswap_64(x)
# define le64toh(x)     (x)
#elif BYTE_ORDER == BIG_ENDIAN
# define be32toh(x)     (x)
# define le32toh(x)     bswap_32(x)
# define be64toh(x)     (x)
# define le64toh(x)     bswap_64(x)
#endif

__attribute__((noreturn))
static void die(const char *msg)
{
        fputs(msg, stderr);
        exit(EXIT_FAILURE);
}

int main(int argc, const char *argv[])
{
        uint64_t entry;
        size_t nread;
        FILE *file;
        union {
                Elf32_Ehdr ehdr32;
                Elf64_Ehdr ehdr64;
        } hdr;

        if (argc != 2)
                die("Usage: elf-entry <elf-file>\n");

        file = fopen(argv[1], "r");
        if (!file) {
                perror("Unable to open input file");
                return EXIT_FAILURE;
        }

        nread = fread(&hdr, 1, sizeof(hdr), file);
        if (nread != sizeof(hdr)) {
                perror("Unable to read input file");
                fclose(file);
                return EXIT_FAILURE;
        }

        if (memcmp(hdr.ehdr32.e_ident, ELFMAG, SELFMAG)) {
                fclose(file);
                die("Input is not an ELF\n");
        }

        switch (hdr.ehdr32.e_ident[EI_CLASS]) {
        case ELFCLASS32:
                switch (hdr.ehdr32.e_ident[EI_DATA]) {
                case ELFDATA2LSB:
                        entry = le32toh(hdr.ehdr32.e_entry);
                        break;
                case ELFDATA2MSB:
                        entry = be32toh(hdr.ehdr32.e_entry);
                        break;
                default:
                        fclose(file);
                        die("Invalid ELF encoding\n");
                }

                /* Sign extend to form a canonical address */
                entry = (int64_t)(int32_t)entry;
                break;

        case ELFCLASS64:
                switch (hdr.ehdr32.e_ident[EI_DATA]) {
                case ELFDATA2LSB:
                        entry = le64toh(hdr.ehdr64.e_entry);
                        break;
                case ELFDATA2MSB:
                        entry = be64toh(hdr.ehdr64.e_entry);
                        break;
                default:
                        fclose(file);
                        die("Invalid ELF encoding\n");
                }
                break;

        default:
                fclose(file);
                die("Invalid ELF class\n");
        }

        printf("0x%016" PRIx64 "\n", entry);
        fclose(file);
        return EXIT_SUCCESS;
}