gen-hyprel.c

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// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright (C) 2020 - Google LLC
 * Author: David Brazdil <dbrazdil@google.com>
 *
 * Generates relocation information used by the kernel to convert
 * absolute addresses in hyp data from kernel VAs to hyp VAs.
 *
 * This is necessary because hyp code is linked into the same binary
 * as the kernel but executes under different memory mappings.
 * If the compiler used absolute addressing, those addresses need to
 * be converted before they are used by hyp code.
 *
 * The input of this program is the relocatable ELF object containing
 * all hyp code/data, not yet linked into vmlinux. Hyp section names
 * should have been prefixed with `.hyp` at this point.
 *
 * The output (printed to stdout) is an assembly file containing
 * an array of 32-bit integers and static relocations that instruct
 * the linker of `vmlinux` to populate the array entries with offsets
 * to positions in the kernel binary containing VAs used by hyp code.
 *
 * Note that dynamic relocations could be used for the same purpose.
 * However, those are only generated if CONFIG_RELOCATABLE=y.
 */
 
#include <elf.h>
#include <endian.h>
#include <errno.h>
#include <fcntl.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
 
#include <generated/autoconf.h>
 
#define HYP_SECTION_PREFIX      ".hyp"
#define HYP_RELOC_SECTION       ".hyp.reloc"
#define HYP_SECTION_SYMBOL_PREFIX   "__hyp_section_"
 
/*
 * AArch64 relocation type constants.
 * Included in case these are not defined in the host toolchain.
 */
#ifndef R_AARCH64_ABS64
#define R_AARCH64_ABS64         257
#endif
#ifndef R_AARCH64_PREL64
#define R_AARCH64_PREL64        260
#endif
#ifndef R_AARCH64_PREL32
#define R_AARCH64_PREL32        261
#endif
#ifndef R_AARCH64_PREL16
#define R_AARCH64_PREL16        262
#endif
#ifndef R_AARCH64_PLT32
#define R_AARCH64_PLT32         314
#endif
#ifndef R_AARCH64_LD_PREL_LO19
#define R_AARCH64_LD_PREL_LO19      273
#endif
#ifndef R_AARCH64_ADR_PREL_LO21
#define R_AARCH64_ADR_PREL_LO21     274
#endif
#ifndef R_AARCH64_ADR_PREL_PG_HI21
#define R_AARCH64_ADR_PREL_PG_HI21  275
#endif
#ifndef R_AARCH64_ADR_PREL_PG_HI21_NC
#define R_AARCH64_ADR_PREL_PG_HI21_NC   276
#endif
#ifndef R_AARCH64_ADD_ABS_LO12_NC
#define R_AARCH64_ADD_ABS_LO12_NC   277
#endif
#ifndef R_AARCH64_LDST8_ABS_LO12_NC
#define R_AARCH64_LDST8_ABS_LO12_NC 278
#endif
#ifndef R_AARCH64_TSTBR14
#define R_AARCH64_TSTBR14       279
#endif
#ifndef R_AARCH64_CONDBR19
#define R_AARCH64_CONDBR19      280
#endif
#ifndef R_AARCH64_JUMP26
#define R_AARCH64_JUMP26        282
#endif
#ifndef R_AARCH64_CALL26
#define R_AARCH64_CALL26        283
#endif
#ifndef R_AARCH64_LDST16_ABS_LO12_NC
#define R_AARCH64_LDST16_ABS_LO12_NC    284
#endif
#ifndef R_AARCH64_LDST32_ABS_LO12_NC
#define R_AARCH64_LDST32_ABS_LO12_NC    285
#endif
#ifndef R_AARCH64_LDST64_ABS_LO12_NC
#define R_AARCH64_LDST64_ABS_LO12_NC    286
#endif
#ifndef R_AARCH64_MOVW_PREL_G0
#define R_AARCH64_MOVW_PREL_G0      287
#endif
#ifndef R_AARCH64_MOVW_PREL_G0_NC
#define R_AARCH64_MOVW_PREL_G0_NC   288
#endif
#ifndef R_AARCH64_MOVW_PREL_G1
#define R_AARCH64_MOVW_PREL_G1      289
#endif
#ifndef R_AARCH64_MOVW_PREL_G1_NC
#define R_AARCH64_MOVW_PREL_G1_NC   290
#endif
#ifndef R_AARCH64_MOVW_PREL_G2
#define R_AARCH64_MOVW_PREL_G2      291
#endif
#ifndef R_AARCH64_MOVW_PREL_G2_NC
#define R_AARCH64_MOVW_PREL_G2_NC   292
#endif
#ifndef R_AARCH64_MOVW_PREL_G3
#define R_AARCH64_MOVW_PREL_G3      293
#endif
#ifndef R_AARCH64_LDST128_ABS_LO12_NC
#define R_AARCH64_LDST128_ABS_LO12_NC   299
#endif
 
/* Global state of the processed ELF. */
static struct {
    const char  *path;
    char        *begin;
    size_t      size;
    Elf64_Ehdr  *ehdr;
    Elf64_Shdr  *sh_table;
    const char  *sh_string;
} elf;
 
#if defined(CONFIG_CPU_LITTLE_ENDIAN)
 
#define elf16toh(x) le16toh(x)
#define elf32toh(x) le32toh(x)
#define elf64toh(x) le64toh(x)
 
#define ELFENDIAN   ELFDATA2LSB
 
#elif defined(CONFIG_CPU_BIG_ENDIAN)
 
#define elf16toh(x) be16toh(x)
#define elf32toh(x) be32toh(x)
#define elf64toh(x) be64toh(x)
 
#define ELFENDIAN   ELFDATA2MSB
 
#else
 
#error PDP-endian sadly unsupported...
 
#endif
 
#define fatal_error(fmt, ...)                       \
    ({                              \
        fprintf(stderr, "error: %s: " fmt "\n",         \
            elf.path, ## __VA_ARGS__);          \
        exit(EXIT_FAILURE);                 \
        __builtin_unreachable();                \
    })
 
#define fatal_perror(msg)                       \
    ({                              \
        fprintf(stderr, "error: %s: " msg ": %s\n",     \
            elf.path, strerror(errno));         \
        exit(EXIT_FAILURE);                 \
        __builtin_unreachable();                \
    })
 
#define assert_op(lhs, rhs, fmt, op)                    \
    ({                              \
        typeof(lhs) _lhs = (lhs);               \
        typeof(rhs) _rhs = (rhs);               \
                                    \
        if (!(_lhs op _rhs)) {                  \
            fatal_error("assertion " #lhs " " #op " " #rhs  \
                " failed (lhs=" fmt ", rhs=" fmt    \
                ", line=%d)", _lhs, _rhs, __LINE__);    \
        }                           \
    })
 
#define assert_eq(lhs, rhs, fmt)    assert_op(lhs, rhs, fmt, ==)
#define assert_ne(lhs, rhs, fmt)    assert_op(lhs, rhs, fmt, !=)
#define assert_lt(lhs, rhs, fmt)    assert_op(lhs, rhs, fmt, <)
#define assert_ge(lhs, rhs, fmt)    assert_op(lhs, rhs, fmt, >=)
 
/*
 * Return a pointer of a given type at a given offset from
 * the beginning of the ELF file.
 */
#define elf_ptr(type, off) ((type *)(elf.begin + (off)))
 
/* Iterate over all sections in the ELF. */
#define for_each_section(var) \
    for (var = elf.sh_table; var < elf.sh_table + elf16toh(elf.ehdr->e_shnum); ++var)
 
/* Iterate over all Elf64_Rela relocations in a given section. */
#define for_each_rela(shdr, var)                    \
    for (var = elf_ptr(Elf64_Rela, elf64toh(shdr->sh_offset));  \
         var < elf_ptr(Elf64_Rela, elf64toh(shdr->sh_offset) + elf64toh(shdr->sh_size)); var++)
 
/* True if a string starts with a given prefix. */
static inline bool starts_with(const char *str, const char *prefix)
{
    return memcmp(str, prefix, strlen(prefix)) == 0;
}
 
/* Returns a string containing the name of a given section. */
static inline const char *section_name(Elf64_Shdr *shdr)
{
    return elf.sh_string + elf32toh(shdr->sh_name);
}
 
/* Returns a pointer to the first byte of section data. */
static inline const char *section_begin(Elf64_Shdr *shdr)
{
    return elf_ptr(char, elf64toh(shdr->sh_offset));
}
 
/* Find a section by its offset from the beginning of the file. */
static inline Elf64_Shdr *section_by_off(Elf64_Off off)
{
    assert_ne(off, 0UL, "%lu");
    return elf_ptr(Elf64_Shdr, off);
}
 
/* Find a section by its index. */
static inline Elf64_Shdr *section_by_idx(uint16_t idx)
{
    assert_ne(idx, SHN_UNDEF, "%u");
    return &elf.sh_table[idx];
}
 
/*
 * Memory-map the given ELF file, perform sanity checks, and
 * populate global state.
 */
static void init_elf(const char *path)
{
    int fd, ret;
    struct stat stat;
 
    /* Store path in the global struct for error printing. */
    elf.path = path;
 
    /* Open the ELF file. */
    fd = open(path, O_RDONLY);
    if (fd < 0)
        fatal_perror("Could not open ELF file");
 
    /* Get status of ELF file to obtain its size. */
    ret = fstat(fd, &stat);
    if (ret < 0) {
        close(fd);
        fatal_perror("Could not get status of ELF file");
    }
 
    /* mmap() the entire ELF file read-only at an arbitrary address. */
    elf.begin = mmap(0, stat.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
    if (elf.begin == MAP_FAILED) {
        close(fd);
        fatal_perror("Could not mmap ELF file");
    }
 
    /* mmap() was successful, close the FD. */
    close(fd);
 
    /* Get pointer to the ELF header. */
    assert_ge(stat.st_size, sizeof(*elf.ehdr), "%lu");
    elf.ehdr = elf_ptr(Elf64_Ehdr, 0);
 
    /* Check the ELF magic. */
    assert_eq(elf.ehdr->e_ident[EI_MAG0], ELFMAG0, "0x%x");
    assert_eq(elf.ehdr->e_ident[EI_MAG1], ELFMAG1, "0x%x");
    assert_eq(elf.ehdr->e_ident[EI_MAG2], ELFMAG2, "0x%x");
    assert_eq(elf.ehdr->e_ident[EI_MAG3], ELFMAG3, "0x%x");
 
    /* Sanity check that this is an ELF64 relocatable object for AArch64. */
    assert_eq(elf.ehdr->e_ident[EI_CLASS], ELFCLASS64, "%u");
    assert_eq(elf.ehdr->e_ident[EI_DATA], ELFENDIAN, "%u");
    assert_eq(elf16toh(elf.ehdr->e_type), ET_REL, "%u");
    assert_eq(elf16toh(elf.ehdr->e_machine), EM_AARCH64, "%u");
 
    /* Populate fields of the global struct. */
    elf.sh_table = section_by_off(elf64toh(elf.ehdr->e_shoff));
    elf.sh_string = section_begin(section_by_idx(elf16toh(elf.ehdr->e_shstrndx)));
}
 
/* Print the prologue of the output ASM file. */
static void emit_prologue(void)
{
    printf(".data\n"
           ".pushsection " HYP_RELOC_SECTION ", \"a\"\n");
}
 
/* Print ASM statements needed as a prologue to a processed hyp section. */
static void emit_section_prologue(const char *sh_orig_name)
{
    /* Declare the hyp section symbol. */
    printf(".global %s%s\n", HYP_SECTION_SYMBOL_PREFIX, sh_orig_name);
}
 
/*
 * Print ASM statements to create a hyp relocation entry for a given
 * R_AARCH64_ABS64 relocation.
 *
 * The linker of vmlinux will populate the position given by `rela` with
 * an absolute 64-bit kernel VA. If the kernel is relocatable, it will
 * also generate a dynamic relocation entry so that the kernel can shift
 * the address at runtime for KASLR.
 *
 * Emit a 32-bit offset from the current address to the position given
 * by `rela`. This way the kernel can iterate over all kernel VAs used
 * by hyp at runtime and convert them to hyp VAs. However, that offset
 * will not be known until linking of `vmlinux`, so emit a PREL32
 * relocation referencing a symbol that the hyp linker script put at
 * the beginning of the relocated section + the offset from `rela`.
 */
static void emit_rela_abs64(Elf64_Rela *rela, const char *sh_orig_name)
{
    /* Offset of this reloc from the beginning of HYP_RELOC_SECTION. */
    static size_t reloc_offset;
 
    /* Create storage for the 32-bit offset. */
    printf(".word 0\n");
 
    /*
     * Create a PREL32 relocation which instructs the linker of `vmlinux`
     * to insert offset to position <base> + <offset>, where <base> is
     * a symbol at the beginning of the relocated section, and <offset>
     * is `rela->r_offset`.
     */
    printf(".reloc %lu, R_AARCH64_PREL32, %s%s + 0x%lx\n",
           reloc_offset, HYP_SECTION_SYMBOL_PREFIX, sh_orig_name,
           elf64toh(rela->r_offset));
 
    reloc_offset += 4;
}
 
/* Print the epilogue of the output ASM file. */
static void emit_epilogue(void)
{
    printf(".popsection\n");
}
 
/*
 * Iterate over all RELA relocations in a given section and emit
 * hyp relocation data for all absolute addresses in hyp code/data.
 *
 * Static relocations that generate PC-relative-addressing are ignored.
 * Failure is reported for unexpected relocation types.
 */
static void emit_rela_section(Elf64_Shdr *sh_rela)
{
    Elf64_Shdr *sh_orig = &elf.sh_table[elf32toh(sh_rela->sh_info)];
    const char *sh_orig_name = section_name(sh_orig);
    Elf64_Rela *rela;
 
    /* Skip all non-hyp sections. */
    if (!starts_with(sh_orig_name, HYP_SECTION_PREFIX))
        return;
 
    emit_section_prologue(sh_orig_name);
 
    for_each_rela(sh_rela, rela) {
        uint32_t type = (uint32_t)elf64toh(rela->r_info);
 
        /* Check that rela points inside the relocated section. */
        assert_lt(elf64toh(rela->r_offset), elf64toh(sh_orig->sh_size), "0x%lx");
 
        switch (type) {
        /*
         * Data relocations to generate absolute addressing.
         * Emit a hyp relocation.
         */
        case R_AARCH64_ABS64:
            emit_rela_abs64(rela, sh_orig_name);
            break;
        /* Allow position-relative data relocations. */
        case R_AARCH64_PREL64:
        case R_AARCH64_PREL32:
        case R_AARCH64_PREL16:
        case R_AARCH64_PLT32:
            break;
        /* Allow relocations to generate PC-relative addressing. */
        case R_AARCH64_LD_PREL_LO19:
        case R_AARCH64_ADR_PREL_LO21:
        case R_AARCH64_ADR_PREL_PG_HI21:
        case R_AARCH64_ADR_PREL_PG_HI21_NC:
        case R_AARCH64_ADD_ABS_LO12_NC:
        case R_AARCH64_LDST8_ABS_LO12_NC:
        case R_AARCH64_LDST16_ABS_LO12_NC:
        case R_AARCH64_LDST32_ABS_LO12_NC:
        case R_AARCH64_LDST64_ABS_LO12_NC:
        case R_AARCH64_LDST128_ABS_LO12_NC:
            break;
        /* Allow relative relocations for control-flow instructions. */
        case R_AARCH64_TSTBR14:
        case R_AARCH64_CONDBR19:
        case R_AARCH64_JUMP26:
        case R_AARCH64_CALL26:
            break;
        /* Allow group relocations to create PC-relative offset inline. */
        case R_AARCH64_MOVW_PREL_G0:
        case R_AARCH64_MOVW_PREL_G0_NC:
        case R_AARCH64_MOVW_PREL_G1:
        case R_AARCH64_MOVW_PREL_G1_NC:
        case R_AARCH64_MOVW_PREL_G2:
        case R_AARCH64_MOVW_PREL_G2_NC:
        case R_AARCH64_MOVW_PREL_G3:
            break;
        default:
            fatal_error("Unexpected RELA type %u", type);
        }
    }
}
 
/* Iterate over all sections and emit hyp relocation data for RELA sections. */
static void emit_all_relocs(void)
{
    Elf64_Shdr *shdr;
 
    for_each_section(shdr) {
        switch (elf32toh(shdr->sh_type)) {
        case SHT_REL:
            fatal_error("Unexpected SHT_REL section \"%s\"",
                section_name(shdr));
        case SHT_RELA:
            emit_rela_section(shdr);
            break;
        }
    }
}
 
int main(int argc, const char **argv)
{
    if (argc != 2) {
        fprintf(stderr, "Usage: %s <elf_input>\n", argv[0]);
        return EXIT_FAILURE;
    }
 
    init_elf(argv[1]);
 
    emit_prologue();
    emit_all_relocs();
    emit_epilogue();
 
    return EXIT_SUCCESS;
}