vDSO reference parser updates and Go workaround

index 8587020..1dbb4b8 100644
@@ -1,6 +1,6 @@
 /*
  * parse_vdso.c: Linux reference vDSO parser
- * Written by Andrew Lutomirski, 2011.
+ * Written by Andrew Lutomirski, 2011-2014.
  *
  * 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

@@ -11,13 +11,14 @@
  * 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
+ * This code is tested on x86.  In principle it should work on any
  * architecture that has a vDSO.
  */
 
 #include <stdbool.h>
 #include <stdint.h>
 #include <string.h>
+#include <limits.h>
 #include <elf.h>
 
 /*

@@ -45,11 +46,18 @@ 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
+#ifndef ELF_BITS
+# if ULONG_MAX > 0xffffffffUL
+#  define ELF_BITS 64
+# else
+#  define ELF_BITS 32
+# endif
 #endif
 
+#define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
+#define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
+#define ELF(x) ELF_BITS_XFORM(ELF_BITS, x)
+
 static struct vdso_info
 {
 	bool valid;

@@ -59,14 +67,14 @@ static struct vdso_info
 	uintptr_t load_offset;  /* load_addr - recorded vaddr */
 
 	/* Symbol table */
-	Elf64_Sym *symtab;
+	ELF(Sym) *symtab;
 	const char *symstrings;
-	Elf64_Word *bucket, *chain;
-	Elf64_Word nbucket, nchain;
+	ELF(Word) *bucket, *chain;
+	ELF(Word) nbucket, nchain;
 
 	/* Version table */
-	Elf64_Versym *versym;
-	Elf64_Verdef *verdef;
+	ELF(Versym) *versym;
+	ELF(Verdef) *verdef;
 } vdso_info;
 
 /* Straight from the ELF specification. */

@@ -92,9 +100,14 @@ void vdso_init_from_sysinfo_ehdr(uintptr_t base)
 
 	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;
+	ELF(Ehdr) *hdr = (ELF(Ehdr)*)base;
+	if (hdr->e_ident[EI_CLASS] !=
+	    (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) {
+		return;  /* Wrong ELF class -- check ELF_BITS */
+	}
+
+	ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff);
+	ELF(Dyn) *dyn = 0;
 
 	/*
 	 * We need two things from the segment table: the load offset

@@ -108,7 +121,7 @@ void vdso_init_from_sysinfo_ehdr(uintptr_t 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);
+			dyn = (ELF(Dyn)*)(base + pt[i].p_offset);
 		}
 	}
 

@@ -118,7 +131,7 @@ void vdso_init_from_sysinfo_ehdr(uintptr_t base)
 	/*
 	 * Fish out the useful bits of the dynamic table.
 	 */
-	Elf64_Word *hash = 0;
+	ELF(Word) *hash = 0;
 	vdso_info.symstrings = 0;
 	vdso_info.symtab = 0;
 	vdso_info.versym = 0;

@@ -131,22 +144,22 @@ void vdso_init_from_sysinfo_ehdr(uintptr_t base)
 				 + vdso_info.load_offset);
 			break;
 		case DT_SYMTAB:
-			vdso_info.symtab = (Elf64_Sym *)
+			vdso_info.symtab = (ELF(Sym) *)
 				((uintptr_t)dyn[i].d_un.d_ptr
 				 + vdso_info.load_offset);
 			break;
 		case DT_HASH:
-			hash = (Elf64_Word *)
+			hash = (ELF(Word) *)
 				((uintptr_t)dyn[i].d_un.d_ptr
 				 + vdso_info.load_offset);
 			break;
 		case DT_VERSYM:
-			vdso_info.versym = (Elf64_Versym *)
+			vdso_info.versym = (ELF(Versym) *)
 				((uintptr_t)dyn[i].d_un.d_ptr
 				 + vdso_info.load_offset);
 			break;
 		case DT_VERDEF:
-			vdso_info.verdef = (Elf64_Verdef *)
+			vdso_info.verdef = (ELF(Verdef) *)
 				((uintptr_t)dyn[i].d_un.d_ptr
 				 + vdso_info.load_offset);
 			break;

@@ -168,8 +181,8 @@ void vdso_init_from_sysinfo_ehdr(uintptr_t base)
 	vdso_info.valid = true;
 }
 
-static bool vdso_match_version(Elf64_Versym ver,
-			       const char *name, Elf64_Word hash)
+static bool vdso_match_version(ELF(Versym) ver,
+			       const char *name, ELF(Word) hash)
 {
 	/*
 	 * This is a helper function to check if the version indexed by

@@ -188,7 +201,7 @@ static bool vdso_match_version(Elf64_Versym ver,
 
 	/* First step: find the version definition */
 	ver &= 0x7fff;  /* Apparently bit 15 means "hidden" */
-	Elf64_Verdef *def = vdso_info.verdef;
+	ELF(Verdef) *def = vdso_info.verdef;
 	while(true) {
 		if ((def->vd_flags & VER_FLG_BASE) == 0
 		    && (def->vd_ndx & 0x7fff) == ver)

@@ -197,11 +210,11 @@ static bool vdso_match_version(Elf64_Versym ver,
 		if (def->vd_next == 0)
 			return false;  /* No definition. */
 
-		def = (Elf64_Verdef *)((char *)def + def->vd_next);
+		def = (ELF(Verdef) *)((char *)def + def->vd_next);
 	}
 
 	/* Now figure out whether it matches. */
-	Elf64_Verdaux *aux = (Elf64_Verdaux*)((char *)def + def->vd_aux);
+	ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux);
 	return def->vd_hash == hash
 		&& !strcmp(name, vdso_info.symstrings + aux->vda_name);
 }

@@ -213,10 +226,10 @@ void *vdso_sym(const char *version, const char *name)
 		return 0;
 
 	ver_hash = elf_hash(version);
-	Elf64_Word chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
+	ELF(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];
+		ELF(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)

@@ -243,7 +256,7 @@ void *vdso_sym(const char *version, const char *name)
 
 void vdso_init_from_auxv(void *auxv)
 {
-	Elf64_auxv_t *elf_auxv = auxv;
+	ELF(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) {

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52 lines not on this page · raw message

index 450ac6e..7a6bf50 100644
@@ -54,7 +54,7 @@ static void fail(const char *format, ...)
 }
 
 /*
- * Evil macros to do a little-endian read.
+ * Evil macros for little-endian reads and writes
  */
 #define GLE(x, bits, ifnot)						\
 	__builtin_choose_expr(						\

@@ -62,11 +62,24 @@ static void fail(const char *format, ...)
 		(__typeof__(*(x)))get_unaligned_le##bits(x), ifnot)
 
 extern void bad_get_le(void);
-#define LAST_LE(x)							\
+#define LAST_GLE(x)							\
 	__builtin_choose_expr(sizeof(*(x)) == 1, *(x), bad_get_le())
 
 #define GET_LE(x)							\
-	GLE(x, 64, GLE(x, 32, GLE(x, 16, LAST_LE(x))))
+	GLE(x, 64, GLE(x, 32, GLE(x, 16, LAST_GLE(x))))
+
+#define PLE(x, val, bits, ifnot)					\
+	__builtin_choose_expr(						\
+		(sizeof(*(x)) == bits/8),				\
+		put_unaligned_le##bits((val), (x)), ifnot)
+
+extern void bad_put_le(void);
+#define LAST_PLE(x, val)						\
+	__builtin_choose_expr(sizeof(*(x)) == 1, *(x) = (val), bad_put_le())
+
+#define PUT_LE(x, val)					\
+	PLE(x, val, 64, PLE(x, val, 32, PLE(x, val, 16, LAST_PLE(x, val))))
+
 
 #define NSYMS (sizeof(required_syms) / sizeof(required_syms[0]))
 

new file mode 100644
index 0000000..cb8a8d7
@@ -0,0 +1,32 @@
+/*
+ * Copyright 2014 Andy Lutomirski
+ * Subject to the GNU Public License, v.2
+ *
+ * Hack to keep broken Go programs working.
+ *
+ * The Go runtime had a couple of bugs: it would read the section table to try
+ * to figure out how many dynamic symbols there were (it shouldn't have looked
+ * at the section table at all) and, if there were no SHT_SYNDYM section table
+ * entry, it would use an uninitialized value for the number of symbols.  As a
+ * workaround, we supply a minimal section table.  vdso2c will adjust the
+ * in-memory image so that "vdso_fake_sections" becomes the section table.
+ *
+ * The bug was introduced by:
+ * https://code.google.com/p/go/source/detail?r=56ea40aac72b (2012-08-31)
+ * and is being addressed in the Go runtime in this issue:
+ * https://code.google.com/p/go/issues/detail?id=8197
+ */
+
+#ifndef __x86_64__
+#error This hack is specific to the 64-bit vDSO
+#endif
+
+#include <linux/elf.h>
+
+extern const __visible struct elf64_shdr vdso_fake_sections[];
+const __visible struct elf64_shdr vdso_fake_sections[] = {
+	{
+		.sh_type = SHT_DYNSYM,
+		.sh_entsize = sizeof(Elf64_Sym),
+	}
+};

index d9f6f61..c6eefaf 100644
@@ -18,6 +18,8 @@ static void GOFUNC(void *addr, size_t len, FILE *outfile, const char *name)
 	const char *secstrings;
 	uint64_t syms[NSYMS] = {};
 
+	uint64_t fake_sections_value = 0, fake_sections_size = 0;
+
 	Elf_Phdr *pt = (Elf_Phdr *)(addr + GET_LE(&hdr->e_phoff));
 
 	/* Walk the segment table. */

@@ -84,6 +86,7 @@ static void GOFUNC(void *addr, size_t len, FILE *outfile, const char *name)
 			GET_LE(&symtab_hdr->sh_entsize) * i;
 		const char *name = addr + GET_LE(&strtab_hdr->sh_offset) +
 			GET_LE(&sym->st_name);
+
 		for (k = 0; k < NSYMS; k++) {
 			if (!strcmp(name, required_syms[k])) {
 				if (syms[k]) {

@@ -93,6 +96,13 @@ static void GOFUNC(void *addr, size_t len, FILE *outfile, const char *name)
 				syms[k] = GET_LE(&sym->st_value);
 			}
 		}
+
+		if (!strcmp(name, "vdso_fake_sections")) {
+			if (fake_sections_value)
+				fail("duplicate vdso_fake_sections\n");
+			fake_sections_value = GET_LE(&sym->st_value);
+			fake_sections_size = GET_LE(&sym->st_size);
+		}
 	}
 
 	/* Validate mapping addresses. */

@@ -112,11 +122,14 @@ static void GOFUNC(void *addr, size_t len, FILE *outfile, const char *name)
 	if (syms[sym_end_mapping] % 4096)
 		fail("end_mapping must be a multiple of 4096\n");
 
-	/* Remove sections. */
-	hdr->e_shoff = 0;
-	hdr->e_shentsize = 0;
-	hdr->e_shnum = 0;
-	hdr->e_shstrndx = SHN_UNDEF; /* SHN_UNDEF == 0 */
+	/* Remove sections or use fakes */
+	if (fake_sections_size % sizeof(Elf_Shdr))
+		fail("vdso_fake_sections size is not a multiple of %ld\n",
+		     (long)sizeof(Elf_Shdr));
+	PUT_LE(&hdr->e_shoff, fake_sections_value);
+	PUT_LE(&hdr->e_shentsize, fake_sections_value ? sizeof(Elf_Shdr) : 0);
+	PUT_LE(&hdr->e_shnum, fake_sections_size / sizeof(Elf_Shdr));
+	PUT_LE(&hdr->e_shstrndx, SHN_UNDEF);
 
 	if (!name) {
 		fwrite(addr, load_size, 1, outfile);