elf: Fix GL(dl_phdr) and GL(dl_phnum) for static builds [BZ #29864]

The 73fc4e28b9 refactor did not add the GL(dl_phdr) and
GL(dl_phnum) for static build, relying on the __ehdr_start symbol,
which is always added by the static linker, to get the correct values.

This is problematic in some ways:

  - The segment may see its in-memory size differ from its in-file
    size (or the binary may have holes).  The Linux has fixed is to
    provide concise values for both AT_PHDR and AT_PHNUM (commit
    0da1d5002745c - "fs/binfmt_elf: Fix AT_PHDR for unusual ELF files")

  - Some archs (alpha for instance) the hidden weak reference is not
    correctly pulled by the static linker and  __ehdr_start address
    end up being 0, which makes GL(dl_phdr) and GL(dl_phnum) have both
    invalid values (and triggering a segfault later on libc.so while
    accessing TLS variables).

The safer fix is to just restore the previous behavior to setup
GL(dl_phdr) and GL(dl_phnum) for static based on kernel auxv.  The
__ehdr_start fallback can also be simplified by not assuming weak
linkage (as for PIE).

The libc-static.c auxv init logic is moved to dl-support.c, since
the later is build without SHARED and then GLRO macro is defined
to access the variables directly.

The _dl_phdr is also assumed to be always non NULL, since an invalid
NULL values does not trigger TLS initialization (which is used in
various libc systems).

Checked on aarch64-linux-gnu, x86_64-linux-gnu, and i686-linux-gnu.

Reviewed-by: Florian Weimer <fweimer@redhat.com>
(cherry picked from commit 7e31d16651)

NEWS

@@ -26,6 +26,8 @@ The following bugs are resolved with this release:
   [12154] Do not fail DNS resolution for CNAMEs which are not host names
   [24816] Fix tst-nss-files-hosts-long on single-stack hosts
   [28846] CMSG_NXTHDR may trigger -Wstrict-overflow warning
+  [29864] libc: __libc_start_main() should obtain program headers
+    address (_dl_phdr) from the auxv, not the ELF header.
   [29305] Conserve NSS buffer space during DNS packet parsing
   [29402] nscd: nscd: No such file or directory
   [29415] nscd: Fix netlink cache invalidation if epoll is used

csu/libc-start.c

@@ -262,28 +262,7 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
   }
 #  endif
   _dl_aux_init (auxvec);
-  if (GL(dl_phdr) == NULL)
 # endif
-    {
-      /* Starting from binutils-2.23, the linker will define the
-         magic symbol __ehdr_start to point to our own ELF header
-         if it is visible in a segment that also includes the phdrs.
-         So we can set up _dl_phdr and _dl_phnum even without any
-         information from auxv.  */
-
-      extern const ElfW(Ehdr) __ehdr_start
-# if BUILD_PIE_DEFAULT
-	__attribute__ ((visibility ("hidden")));
-# else
-	__attribute__ ((weak, visibility ("hidden")));
-      if (&__ehdr_start != NULL)
-# endif
-        {
-          assert (__ehdr_start.e_phentsize == sizeof *GL(dl_phdr));
-          GL(dl_phdr) = (const void *) &__ehdr_start + __ehdr_start.e_phoff;
-          GL(dl_phnum) = __ehdr_start.e_phnum;
-        }
-    }
 
   __tunables_init (__environ);
 

csu/libc-tls.c

@@ -118,19 +118,18 @@ __libc_setup_tls (void)
   __tls_pre_init_tp ();
 
   /* Look through the TLS segment if there is any.  */
-  if (_dl_phdr != NULL)
-    for (phdr = _dl_phdr; phdr < &_dl_phdr[_dl_phnum]; ++phdr)
-      if (phdr->p_type == PT_TLS)
-	{
-	  /* Remember the values we need.  */
-	  memsz = phdr->p_memsz;
-	  filesz = phdr->p_filesz;
-	  initimage = (void *) phdr->p_vaddr + main_map->l_addr;
-	  align = phdr->p_align;
-	  if (phdr->p_align > max_align)
-	    max_align = phdr->p_align;
-	  break;
-	}
+  for (phdr = _dl_phdr; phdr < &_dl_phdr[_dl_phnum]; ++phdr)
+    if (phdr->p_type == PT_TLS)
+      {
+	/* Remember the values we need.  */
+	memsz = phdr->p_memsz;
+	filesz = phdr->p_filesz;
+	initimage = (void *) phdr->p_vaddr + main_map->l_addr;
+	align = phdr->p_align;
+	if (phdr->p_align > max_align)
+	  max_align = phdr->p_align;
+	break;
+      }
 
   /* Calculate the size of the static TLS surplus, with 0 auditors.  */
   _dl_tls_static_surplus_init (0);

elf/dl-support.c

@@ -255,6 +255,25 @@ _dl_aux_init (ElfW(auxv_t) *av)
   for (int i = 0; i < array_length (auxv_values); ++i)
     auxv_values[i] = 0;
   _dl_parse_auxv (av, auxv_values);
+
+  _dl_phdr = (void*) auxv_values[AT_PHDR];
+  _dl_phnum = auxv_values[AT_PHNUM];
+
+  if (_dl_phdr == NULL)
+    {
+      /* Starting from binutils-2.23, the linker will define the
+         magic symbol __ehdr_start to point to our own ELF header
+         if it is visible in a segment that also includes the phdrs.
+         So we can set up _dl_phdr and _dl_phnum even without any
+         information from auxv.  */
+
+      extern const ElfW(Ehdr) __ehdr_start attribute_hidden;
+      assert (__ehdr_start.e_phentsize == sizeof *GL(dl_phdr));
+      _dl_phdr = (const void *) &__ehdr_start + __ehdr_start.e_phoff;
+      _dl_phnum = __ehdr_start.e_phnum;
+    }
+
+  assert (_dl_phdr != NULL);
 }
 #endif
 
@@ -323,20 +342,19 @@ _dl_non_dynamic_init (void)
   if (_dl_platform != NULL)
     _dl_platformlen = strlen (_dl_platform);
 
-  if (_dl_phdr != NULL)
-    for (const ElfW(Phdr) *ph = _dl_phdr; ph < &_dl_phdr[_dl_phnum]; ++ph)
-      switch (ph->p_type)
-	{
-	/* Check if the stack is nonexecutable.  */
-	case PT_GNU_STACK:
-	  _dl_stack_flags = ph->p_flags;
-	  break;
+  for (const ElfW(Phdr) *ph = _dl_phdr; ph < &_dl_phdr[_dl_phnum]; ++ph)
+    switch (ph->p_type)
+      {
+      /* Check if the stack is nonexecutable.  */
+      case PT_GNU_STACK:
+	_dl_stack_flags = ph->p_flags;
+	break;
 
-	case PT_GNU_RELRO:
-	  _dl_main_map.l_relro_addr = ph->p_vaddr;
-	  _dl_main_map.l_relro_size = ph->p_memsz;
-	  break;
-	}
+      case PT_GNU_RELRO:
+	_dl_main_map.l_relro_addr = ph->p_vaddr;
+	_dl_main_map.l_relro_size = ph->p_memsz;
+	break;
+      }
 
   call_function_static_weak (_dl_find_object_init);