/*
   * parse_vdso.c: Linux reference vDSO parser
   * Written by Andrew Lutomirski, 2011.
   *
   * This code is meant to be linked in to various programs that run on Linux.
   * As such, it is available with as few restrictions as possible.  This file
   * is licensed under the Creative Commons Zero License, version 1.0,
   * available at http://creativecommons.org/publicdomain/zero/1.0/legalcode
   *
   * The vDSO is a regular ELF DSO that the kernel maps into user space when
   * it starts a program.  It works equally well in statically and dynamically
   * linked binaries.
   *
   * This code is tested on x86_64.  In principle it should work on any 64-bit
   * architecture that has a vDSO.
   */
  
  #include <stdbool.h>
  #include <stdint.h>
  #include <string.h>
  #include <elf.h>
  
  /*
   * To use this vDSO parser, first call one of the vdso_init_* functions.
   * If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR
   * to vdso_init_from_sysinfo_ehdr.  Otherwise pass auxv to vdso_init_from_auxv.
   * Then call vdso_sym for each symbol you want.  For example, to look up
   * gettimeofday on x86_64, use:
   *
   *     <some pointer> = vdso_sym("LINUX_2.6", "gettimeofday");
   * or
   *     <some pointer> = vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
   *
   * vdso_sym will return 0 if the symbol doesn't exist or if the init function
   * failed or was not called.  vdso_sym is a little slow, so its return value
   * should be cached.
   *
   * vdso_sym is threadsafe; the init functions are not.
   *
   * These are the prototypes:
   */
  extern void vdso_init_from_auxv(void *auxv);
  extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
  extern void *vdso_sym(const char *version, const char *name);
  
  
  /* And here's the code. */
  
  #ifndef __x86_64__
  # error Not yet ported to non-x86_64 architectures
  #endif
  
  static struct vdso_info
  {
  	bool valid;
  
  	/* Load information */
  	uintptr_t load_addr;
  	uintptr_t load_offset;  /* load_addr - recorded vaddr */
  
  	/* Symbol table */
  	Elf64_Sym *symtab;
  	const char *symstrings;
  	Elf64_Word *bucket, *chain;
  	Elf64_Word nbucket, nchain;
  
  	/* Version table */
  	Elf64_Versym *versym;
  	Elf64_Verdef *verdef;
  } vdso_info;
  
  /* Straight from the ELF specification. */
  static unsigned long elf_hash(const unsigned char *name)
  {
  	unsigned long h = 0, g;
  	while (*name)
  	{
  		h = (h << 4) + *name++;
  		if (g = h & 0xf0000000)
  			h ^= g >> 24;
  		h &= ~g;
  	}
  	return h;
  }
  
  void vdso_init_from_sysinfo_ehdr(uintptr_t base)
  {
  	size_t i;
  	bool found_vaddr = false;
  
  	vdso_info.valid = false;
  
  	vdso_info.load_addr = base;
  
  	Elf64_Ehdr *hdr = (Elf64_Ehdr*)base;
  	Elf64_Phdr *pt = (Elf64_Phdr*)(vdso_info.load_addr + hdr->e_phoff);
  	Elf64_Dyn *dyn = 0;
  
  	/*
  	 * We need two things from the segment table: the load offset
  	 * and the dynamic table.
  	 */
  	for (i = 0; i < hdr->e_phnum; i++)
  	{
  		if (pt[i].p_type == PT_LOAD && !found_vaddr) {
  			found_vaddr = true;
  			vdso_info.load_offset =	base
  				+ (uintptr_t)pt[i].p_offset
  				- (uintptr_t)pt[i].p_vaddr;
  		} else if (pt[i].p_type == PT_DYNAMIC) {
  			dyn = (Elf64_Dyn*)(base + pt[i].p_offset);
  		}
  	}
  
  	if (!found_vaddr || !dyn)
  		return;  /* Failed */
  
  	/*
  	 * Fish out the useful bits of the dynamic table.
  	 */
  	Elf64_Word *hash = 0;
  	vdso_info.symstrings = 0;
  	vdso_info.symtab = 0;
  	vdso_info.versym = 0;
  	vdso_info.verdef = 0;
  	for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
  		switch (dyn[i].d_tag) {
  		case DT_STRTAB:
  			vdso_info.symstrings = (const char *)
  				((uintptr_t)dyn[i].d_un.d_ptr
  				 + vdso_info.load_offset);
  			break;
  		case DT_SYMTAB:
  			vdso_info.symtab = (Elf64_Sym *)
  				((uintptr_t)dyn[i].d_un.d_ptr
  				 + vdso_info.load_offset);
  			break;
  		case DT_HASH:
  			hash = (Elf64_Word *)
  				((uintptr_t)dyn[i].d_un.d_ptr
  				 + vdso_info.load_offset);
  			break;
  		case DT_VERSYM:
  			vdso_info.versym = (Elf64_Versym *)
  				((uintptr_t)dyn[i].d_un.d_ptr
  				 + vdso_info.load_offset);
  			break;
  		case DT_VERDEF:
  			vdso_info.verdef = (Elf64_Verdef *)
  				((uintptr_t)dyn[i].d_un.d_ptr
  				 + vdso_info.load_offset);
  			break;
  		}
  	}
  	if (!vdso_info.symstrings || !vdso_info.symtab || !hash)
  		return;  /* Failed */
  
  	if (!vdso_info.verdef)
  		vdso_info.versym = 0;
  
  	/* Parse the hash table header. */
  	vdso_info.nbucket = hash[0];
  	vdso_info.nchain = hash[1];
  	vdso_info.bucket = &hash[2];
  	vdso_info.chain = &hash[vdso_info.nbucket + 2];
  
  	/* That's all we need. */
  	vdso_info.valid = true;
  }
  
  static bool vdso_match_version(Elf64_Versym ver,
  			       const char *name, Elf64_Word hash)
  {
  	/*
  	 * This is a helper function to check if the version indexed by
  	 * ver matches name (which hashes to hash).
  	 *
  	 * The version definition table is a mess, and I don't know how
  	 * to do this in better than linear time without allocating memory
  	 * to build an index.  I also don't know why the table has
  	 * variable size entries in the first place.
  	 *
  	 * For added fun, I can't find a comprehensible specification of how
  	 * to parse all the weird flags in the table.
  	 *
  	 * So I just parse the whole table every time.
  	 */
  
  	/* First step: find the version definition */
  	ver &= 0x7fff;  /* Apparently bit 15 means "hidden" */
  	Elf64_Verdef *def = vdso_info.verdef;
  	while(true) {
  		if ((def->vd_flags & VER_FLG_BASE) == 0
  		    && (def->vd_ndx & 0x7fff) == ver)
  			break;
  
  		if (def->vd_next == 0)
  			return false;  /* No definition. */
  
  		def = (Elf64_Verdef *)((char *)def + def->vd_next);
  	}
  
  	/* Now figure out whether it matches. */
  	Elf64_Verdaux *aux = (Elf64_Verdaux*)((char *)def + def->vd_aux);
  	return def->vd_hash == hash
  		&& !strcmp(name, vdso_info.symstrings + aux->vda_name);
  }
  
  void *vdso_sym(const char *version, const char *name)
  {
  	unsigned long ver_hash;
  	if (!vdso_info.valid)
  		return 0;
  
  	ver_hash = elf_hash(version);
  	Elf64_Word chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
  
  	for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
  		Elf64_Sym *sym = &vdso_info.symtab[chain];
  
  		/* Check for a defined global or weak function w/ right name. */
  		if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
  			continue;
  		if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
  		    ELF64_ST_BIND(sym->st_info) != STB_WEAK)
  			continue;
  		if (sym->st_shndx == SHN_UNDEF)
  			continue;
  		if (strcmp(name, vdso_info.symstrings + sym->st_name))
  			continue;
  
  		/* Check symbol version. */
  		if (vdso_info.versym
  		    && !vdso_match_version(vdso_info.versym[chain],
  					   version, ver_hash))
  			continue;
  
  		return (void *)(vdso_info.load_offset + sym->st_value);
  	}
  
  	return 0;
  }
  
  void vdso_init_from_auxv(void *auxv)
  {
  	Elf64_auxv_t *elf_auxv = auxv;
  	for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
  	{
  		if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
  			vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val);
  			return;
  		}
  	}
  
  	vdso_info.valid = false;
  }