firmware: add simple ELF loader for static binaries

commit a610998ea4544bf25d2562268a866163c70b0d59
Author: Aidan MacDonald <[email protected]>
Date:   Sun Jan 4 00:00:05 2026 +0000

    firmware: add simple ELF loader for static binaries
    
    This is a small & simple ELF loader which just copies
    program segments from disk to memory. It only supports
    static binaries right now.
    
    Change-Id: I8944feb5b9dafcc5c56e12383aed25e2718ad7ea

diff --git a/firmware/SOURCES b/firmware/SOURCES
index 9279307af5..88927f7364 100644
--- a/firmware/SOURCES
+++ b/firmware/SOURCES
@@ -58,6 +58,10 @@ target/hosted/rolo.c
 lc-rock.c
 #endif
 
+#if defined(HAVE_ELF)
+elf_loader.c
+#endif
+
 #if defined(HAVE_BOOTDATA) || defined(HAVE_MULTIBOOT)
 common/multiboot.c
 #ifndef BOOTLOADER
diff --git a/firmware/elf_loader.c b/firmware/elf_loader.c
new file mode 100644
index 0000000000..d6fd2e71d4
--- /dev/null
+++ b/firmware/elf_loader.c
@@ -0,0 +1,175 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2026 Aidan MacDonald
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+#include "elf_loader.h"
+#include "elf-target.h"
+#include "file.h"
+
+#if defined(ROCKBOX_LITTLE_ENDIAN)
+# define NATIVE_ELF_DATA ELFDATA2LSB
+#elif defined(ROCKBOX_BIG_ENDIAN)
+# define NATIVE_ELF_DATA ELFDATA2MSB
+#else
+# error "Unknown endianness!"
+#endif
+
+_Static_assert(TARGET_ELF_DATA == NATIVE_ELF_DATA,
+               "ELF binaries with foreign endianness not supported");
+
+static int elf_validate_header(const struct elf_header *header)
+{
+    if (header->ei_magic[0] != ELFMAG0 ||
+        header->ei_magic[1] != ELFMAG1 ||
+        header->ei_magic[2] != ELFMAG2 ||
+        header->ei_magic[3] != ELFMAG3)
+        return ELF_ERR_BADMAGIC;
+
+    if (header->ei_class != TARGET_ELF_CLASS ||
+        header->ei_data != TARGET_ELF_DATA)
+        return ELF_ERR_UNSUPP_ARCH;
+
+    if (header->ei_version != EV_CURRENT ||
+        header->e_version != EV_CURRENT)
+        return ELF_ERR_UNSUPP_VERSION;
+
+    if (header->ei_osabi != TARGET_ELF_OSABI ||
+        header->ei_abiversion != TARGET_ELF_ABIVERSION)
+        return ELF_ERR_UNSUPP_OS;
+
+    if (header->e_type != ET_EXEC)
+        return ELF_ERR_UNSUPP_TYPE;
+
+    if (header->e_machine != TARGET_ELF_MACHINE)
+        return ELF_ERR_UNSUPP_ARCH;
+
+    if (header->e_phoff == 0)
+        return ELF_ERR_EMPTY;
+
+    if (header->e_phentsize < sizeof(struct elf_phdr))
+        return ELF_ERR_UNSUPP_PHSIZE;
+
+    if (header->e_phnum == PN_XNUM)
+        return ELF_ERR_TOO_MANY_PHDRS;
+
+    return ELF_OK;
+}
+
+static int elf_validate_phdr(const struct elf_phdr *phdr,
+                             const struct elf_load_context *ctx)
+{
+    if (phdr->p_type != PT_LOAD)
+        return ELF_ERR_UNSUPP_PHDR;
+
+    if (phdr->p_filesz > phdr->p_memsz)
+        return ELF_ERR_INVALID_PHDR;
+
+    /* Ignore headers which occupy zero memory */
+    if (phdr->p_memsz == 0)
+        return 0;
+
+    for (size_t i = 0; i < ctx->num_mmap; ++i)
+    {
+        const struct elf_memory_map *entry = &ctx->mmap[i];
+        if (phdr->p_vaddr >= entry->addr &&
+            phdr->p_vaddr  < entry->addr + entry->size &&
+            phdr->p_memsz <= entry->size)
+        {
+            if ((phdr->p_flags & entry->flags) != phdr->p_flags)
+                return ELF_ERR_MEM_INCOMPAT_FLAGS;
+
+            return ELF_OK;
+        }
+    }
+
+    return ELF_ERR_MEM_UNMAPPED;
+}
+
+int elf_loadfd(int fd,
+               const struct elf_load_context *ctx,
+               void **entrypoint)
+{
+    struct elf_header ehdr;
+    struct elf_phdr phdr;
+    ssize_t nread;
+    int err;
+
+    nread = read(fd, &ehdr, sizeof(ehdr));
+    if (nread != (ssize_t)sizeof(ehdr))
+        return ELF_ERR_IO;
+
+    err = elf_validate_header(&ehdr);
+    if (err != ELF_OK)
+        return err;
+
+    /*
+     * Iterate over program headers, copying segments to memory
+     */
+    for (size_t ph_index = 0; ph_index < ehdr.e_phnum; ++ph_index)
+    {
+        off_t ph_off = ehdr.e_phoff + (ph_index * ehdr.e_phentsize);
+        if (lseek(fd, ph_off, SEEK_SET) == (off_t)-1)
+            return ELF_ERR_IO;
+
+        nread = read(fd, &phdr, sizeof(phdr));
+        if (nread != (ssize_t)sizeof(phdr))
+            return ELF_ERR_IO;
+
+        err = elf_validate_phdr(&phdr, ctx);
+        if (err)
+            return err;
+
+        /* Load file data to memory if needed */
+        if (phdr.p_filesz > 0)
+        {
+            if (lseek(fd, phdr.p_offset, SEEK_SET) == (off_t)-1)
+                return ELF_ERR_IO;
+
+            nread = read(fd, (void *)phdr.p_vaddr, phdr.p_filesz);
+            if (nread < 0 || (uint32_t)nread != phdr.p_filesz)
+                return ELF_ERR_IO;
+        }
+
+        /* Zero out any memory not backed by file data */
+        if (phdr.p_memsz > phdr.p_filesz)
+        {
+            void *addr = (void *)phdr.p_vaddr + phdr.p_filesz;
+            size_t size = phdr.p_memsz - phdr.p_filesz;
+
+            memset(addr, 0, size);
+        }
+    }
+
+    *entrypoint = (void *)ehdr.e_entry;
+    return ELF_OK;
+}
+
+int elf_loadpath(const char *filename,
+                 const struct elf_load_context *ctx,
+                 void **entrypoint)
+{
+    int fd = open(filename, O_RDONLY);
+    if (fd < 0)
+        return ELF_ERR_FILE_NOT_FOUND;
+
+    int err = elf_loadfd(fd, ctx, entrypoint);
+
+    close(fd);
+    return err;
+}
diff --git a/firmware/include/elf-target.h b/firmware/include/elf-target.h
new file mode 100644
index 0000000000..e718cf666b
--- /dev/null
+++ b/firmware/include/elf-target.h
@@ -0,0 +1,50 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2026 Aidan MacDonald
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+#ifndef __ELF_TARGET_H__
+#define __ELF_TARGET_H__
+
+#include "elf.h"
+#include "config.h"
+
+#define TARGET_ELF_CLASS        ELFCLASS32
+#define TARGET_ELF_OSABI        ELFOSABI_SYSV
+#define TARGET_ELF_ABIVERSION   0
+#define elf_phdr                elf32_phdr
+
+#if defined(CPU_ARM)
+# define TARGET_ELF_MACHINE     EM_ARM
+#elif defined(CPU_MIPS)
+# define TARGET_ELF_MACHINE     EM_MIPS
+#elif defined(CPU_COLDFIRE)
+# define TARGET_ELF_MACHINE     EM_68K
+#else
+# error "Unknown CPU architecture!"
+#endif
+
+#if defined(ROCKBOX_LITTLE_ENDIAN)
+# define TARGET_ELF_DATA        ELFDATA2LSB
+#elif defined(ROCKBOX_BIG_ENDIAN)
+# define TARGET_ELF_DATA        ELFDATA2MSB
+#else
+# error "Unknown endianness!"
+#endif
+
+#endif /* __ELF_TARGET_H__ */
diff --git a/firmware/include/elf.h b/firmware/include/elf.h
new file mode 100644
index 0000000000..bfdd4f2281
--- /dev/null
+++ b/firmware/include/elf.h
@@ -0,0 +1,143 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2026 Aidan MacDonald
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+#ifndef __ELF_H__
+#define __ELF_H__
+
+#include <stdint.h>
+
+/* Field offsets in the e_ident array */
+#define EI_MAG0         0
+#define EI_MAG1         1
+#define EI_MAG2         2
+#define EI_MAG3         3
+#define EI_CLASS        4
+#define EI_DATA         5
+#define EI_VERSION      6
+#define EI_OSABI        7
+#define EI_ABIVERSION   8
+#define EI_PAD          9
+#define EI_NIDENT       16
+
+/* Values for ei_magic */
+#define ELFMAG0         0x7f
+#define ELFMAG1         'E'
+#define ELFMAG2         'L'
+#define ELFMAG3         'F'
+
+/* Values for ei_class */
+#define ELFCLASSNONE    0
+#define ELFCLASS32      1
+#define ELFCLASS64      2
+
+/* Values for ei_data */
+#define ELFDATANONE     0
+#define ELFDATA2LSB     1
+#define ELFDATA2MSB     2
+
+/* Values for ei_version and e_version */
+#define EV_NONE         0
+#define EV_CURRENT      1
+
+/* Values for ei_osabi */
+#define ELFOSABI_NONE   ELFOSABI_SYSV
+#define ELFOSABI_SYSV   0
+
+/* Values for e_type */
+#define ET_NONE         0
+#define ET_REL          1
+#define ET_EXEC         2
+#define ET_DYN          3
+#define ET_CORE         4
+
+/* Values for e_machine */
+#define EM_NONE         0
+#define EM_68K          4
+#define EM_MIPS         8
+#define EM_ARM          40
+
+/*
+ * Special value for e_phnum when the real number of
+ * program headers is too large.
+ */
+#define PN_XNUM         0xFFFFu
+
+/* ELF basic types */
+typedef uint32_t elf_addr_t;
+typedef uint32_t elf_off_t;
+
+/* ELF file header */
+struct elf_header
+{
+    union {
+        uint8_t e_ident[EI_NIDENT];
+        struct {
+            uint8_t ei_magic[4];
+            uint8_t ei_class;
+            uint8_t ei_data;
+            uint8_t ei_version;
+            uint8_t ei_osabi;
+            uint8_t ei_abiversion;
+        };
+    };
+
+    uint16_t    e_type;
+    uint16_t    e_machine;
+    uint32_t    e_version;
+    elf_addr_t  e_entry;
+    elf_off_t   e_phoff;
+    elf_off_t   e_shoff;
+    uint32_t    e_flags;
+    uint16_t    e_ehsize;
+    uint16_t    e_phentsize;
+    uint16_t    e_phnum;
+    uint16_t    e_shentsize;
+    uint16_t    e_shnum;
+    uint16_t    e_shstrndx;
+};
+
+/* Values for p_type */
+#define PT_NULL         0
+#define PT_LOAD         1
+#define PT_DYNAMIC      2
+#define PT_INTERP       3
+#define PT_NOTE         4
+#define PT_SHLIB        5
+#define PT_PHDR         6
+
+/* Values for p_flags */
+#define PF_X            (1 << 0)  /* Segment is executable */
+#define PF_W            (1 << 1)  /* Segment is writable */
+#define PF_R            (1 << 2)  /* Segment is readable */
+
+/* ELF 32-bit program header */
+struct elf32_phdr
+{
+    uint32_t    p_type;
+    elf_off_t   p_offset;
+    elf_addr_t  p_vaddr;
+    elf_addr_t  p_paddr;
+    uint32_t    p_filesz;
+    uint32_t    p_memsz;
+    uint32_t    p_flags;
+    uint32_t    p_align;
+};
+
+#endif /* __ELF_H__ */
diff --git a/firmware/include/elf_loader.h b/firmware/include/elf_loader.h
new file mode 100644
index 0000000000..0f30e87ab2
--- /dev/null
+++ b/firmware/include/elf_loader.h
@@ -0,0 +1,94 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2026 Aidan MacDonald
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+#ifndef __ELF_LOADER_H__
+#define __ELF_LOADER_H__
+
+/*
+ * Primitive ELF loader -- allows loading an ELF binary from
+ * a file descriptor into RAM. Only static binaries linked at
+ * a fixed address are supported.
+ *
+ * The caller supplies a memory map which limits where the
+ * ELF file can be loaded; if a program header requests any
+ * memory outside of this mapping, loading will fail. There
+ * is no support for relocation, PIC code, or any form of
+ * dynamic linking.
+ *
+ * The loader can return the entry point of the ELF binary.
+ * It does not support arbitrary symbol lookups.
+ */
+
+#include <stdint.h>
+#include <stddef.h>
+
+enum elf_error
+{
+    ELF_OK,
+    ELF_ERR_FILE_NOT_FOUND,
+    ELF_ERR_IO,
+    ELF_ERR_EMPTY,
+    ELF_ERR_BADMAGIC,
+    ELF_ERR_UNSUPP_ARCH,
+    ELF_ERR_UNSUPP_OS,
+    ELF_ERR_UNSUPP_VERSION,
+    ELF_ERR_UNSUPP_TYPE,
+    ELF_ERR_UNSUPP_FORMAT,
+    ELF_ERR_UNSUPP_PHSIZE,
+    ELF_ERR_TOO_MANY_PHDRS,
+    ELF_ERR_UNSUPP_PHDR,
+    ELF_ERR_INVALID_PHDR,
+    ELF_ERR_MEM_INCOMPAT_FLAGS,
+    ELF_ERR_MEM_UNMAPPED,
+};
+
+/*
+ * Describes an entry in the virtual memory map specified
+ * when loading an ELF file.
+ *
+ * ELF segments must fit completely within a single entry
+ * in the memory map. Segments crossing multiple mappings
+ * are rejected even if the mappings are contiguous.
+ *
+ * A segment may also be rejected if it has flags (R/W/X)
+ * not present in the corresponding memory map entry.
+ */
+struct elf_memory_map
+{
+    uintptr_t addr;
+    size_t    size;
+    uint32_t  flags;
+};
+
+struct elf_load_context
+{
+    const struct elf_memory_map *mmap;
+    size_t num_mmap;
+};
+
+int elf_loadfd(int fd,
+               const struct elf_load_context *ctx,
+               void **entrypoint);
+
+int elf_loadpath(const char *filename,
+                 const struct elf_load_context *ctx,
+                 void **entrypoint);
+
+#endif /* __ELF_LOADER_H__ */
-- 
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