ld.so: Replace DL_RO_DYN_SECTION with dl_relocate_ld [BZ #28340]

We can't relocate entries in dynamic section if it is readonly:

1. Add a l_ld_readonly field to struct link_map to indicate if dynamic
section is readonly and set it based on p_flags of PT_DYNAMIC segment.
2. Replace DL_RO_DYN_SECTION with dl_relocate_ld to decide if dynamic
section should be relocated.
3. Remove DL_RO_DYN_TEMP_CNT.
4. Don't use a static dynamic section to make readonly dynamic section
in vDSO writable.
5. Remove the temp argument from elf_get_dynamic_info.

This fixes BZ #28340.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

elf/Makefile

@@ -224,7 +224,7 @@ tests += restest1 preloadtest loadfail multiload origtest resolvfail \
 	 tst-tls-ie tst-tls-ie-dlmopen argv0test \
 	 tst-glibc-hwcaps tst-glibc-hwcaps-prepend tst-glibc-hwcaps-mask \
 	 tst-tls20 tst-tls21 tst-dlmopen-dlerror tst-dlmopen-gethostbyname \
-	 tst-dl-is_dso
+	 tst-dl-is_dso tst-ro-dynamic
 #	 reldep9
 tests-internal += loadtest unload unload2 circleload1 \
 	 neededtest neededtest2 neededtest3 neededtest4 \
@@ -360,7 +360,7 @@ modules-names = testobj1 testobj2 testobj3 testobj4 testobj5 testobj6 \
 		libmarkermod4-1 libmarkermod4-2 libmarkermod4-3 libmarkermod4-4 \
 		tst-tls20mod-bad tst-tls21mod tst-dlmopen-dlerror-mod \
 		tst-auxvalmod \
-		tst-dlmopen-gethostbyname-mod \
+		tst-dlmopen-gethostbyname-mod tst-ro-dynamic-mod \
 
 # Most modules build with _ISOMAC defined, but those filtered out
 # depend on internal headers.
@@ -1911,3 +1911,10 @@ $(objpfx)tst-getauxval-static.out: $(objpfx)tst-auxvalmod.so
 tst-getauxval-static-ENV = LD_LIBRARY_PATH=$(objpfx):$(common-objpfx)
 
 $(objpfx)tst-dlmopen-gethostbyname.out: $(objpfx)tst-dlmopen-gethostbyname-mod.so
+
+$(objpfx)tst-ro-dynamic: $(objpfx)tst-ro-dynamic-mod.so
+$(objpfx)tst-ro-dynamic-mod.so: $(objpfx)tst-ro-dynamic-mod.os \
+  tst-ro-dynamic-mod.map
+	$(LINK.o) -nostdlib -nostartfiles -shared -o $@ \
+		-Wl,--script=tst-ro-dynamic-mod.map \
+		$(objpfx)tst-ro-dynamic-mod.os

elf/dl-load.c

@@ -1149,6 +1149,7 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
 		 such a segment to avoid a crash later.  */
 	      l->l_ld = (void *) ph->p_vaddr;
 	      l->l_ldnum = ph->p_memsz / sizeof (ElfW(Dyn));
+	      l->l_ld_readonly = (ph->p_flags & PF_W) == 0;
 	    }
 	  break;
 
@@ -1292,7 +1293,7 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
   else
     l->l_ld = (ElfW(Dyn) *) ((ElfW(Addr)) l->l_ld + l->l_addr);
 
-  elf_get_dynamic_info (l, NULL);
+  elf_get_dynamic_info (l);
 
   /* Make sure we are not dlopen'ing an object that has the
      DF_1_NOOPEN flag set, or a PIE object.  */

elf/dl-reloc-static-pie.c

@@ -40,7 +40,17 @@ _dl_relocate_static_pie (void)
 
   /* Read our own dynamic section and fill in the info array.  */
   main_map->l_ld = ((void *) main_map->l_addr + elf_machine_dynamic ());
-  elf_get_dynamic_info (main_map, NULL);
+
+  const ElfW(Phdr) *ph, *phdr = GL(dl_phdr);
+  size_t phnum = GL(dl_phnum);
+  for (ph = phdr; ph < &phdr[phnum]; ++ph)
+    if (ph->p_type == PT_DYNAMIC)
+      {
+	main_map->l_ld_readonly = (ph->p_flags & PF_W) == 0;
+	break;
+      }
+
+  elf_get_dynamic_info (main_map);
 
 # ifdef ELF_MACHINE_BEFORE_RTLD_RELOC
   ELF_MACHINE_BEFORE_RTLD_RELOC (main_map->l_info);

elf/get-dynamic-info.h

@@ -28,7 +28,7 @@ static
 auto
 #endif
 inline void __attribute__ ((unused, always_inline))
-elf_get_dynamic_info (struct link_map *l, ElfW(Dyn) *temp)
+elf_get_dynamic_info (struct link_map *l)
 {
 #if __ELF_NATIVE_CLASS == 32
   typedef Elf32_Word d_tag_utype;
@@ -69,28 +69,15 @@ elf_get_dynamic_info (struct link_map *l, ElfW(Dyn) *temp)
       info[i] = dyn;
     }
 
-#define DL_RO_DYN_TEMP_CNT	8
-
-#ifndef DL_RO_DYN_SECTION
   /* Don't adjust .dynamic unnecessarily.  */
-  if (l->l_addr != 0)
+  if (l->l_addr != 0 && dl_relocate_ld (l))
     {
       ElfW(Addr) l_addr = l->l_addr;
-      int cnt = 0;
 
 # define ADJUST_DYN_INFO(tag) \
       do								      \
 	if (info[tag] != NULL)						      \
-	  {								      \
-	    if (temp)							      \
-	      {								      \
-		temp[cnt].d_tag = info[tag]->d_tag;			      \
-		temp[cnt].d_un.d_ptr = info[tag]->d_un.d_ptr + l_addr;	      \
-		info[tag] = temp + cnt++;				      \
-	      }								      \
-	    else							      \
          info[tag]->d_un.d_ptr += l_addr;				      \
-	  }								      \
       while (0)
 
       ADJUST_DYN_INFO (DT_HASH);
@@ -107,9 +94,7 @@ elf_get_dynamic_info (struct link_map *l, ElfW(Dyn) *temp)
       ADJUST_DYN_INFO (VERSYMIDX (DT_VERSYM));
       ADJUST_DYN_INFO (ADDRIDX (DT_GNU_HASH));
 # undef ADJUST_DYN_INFO
-      assert (cnt <= DL_RO_DYN_TEMP_CNT);
     }
-#endif
   if (info[DT_PLTREL] != NULL)
     {
 #if ELF_MACHINE_NO_RELA

elf/rtld.c

@@ -463,6 +463,7 @@ _dl_start_final (void *arg, struct dl_start_final_info *info)
 #ifndef DONT_USE_BOOTSTRAP_MAP
   GL(dl_rtld_map).l_addr = info->l.l_addr;
   GL(dl_rtld_map).l_ld = info->l.l_ld;
+  GL(dl_rtld_map).l_ld_readonly = info->l.l_ld_readonly;
   memcpy (GL(dl_rtld_map).l_info, info->l.l_info,
 	  sizeof GL(dl_rtld_map).l_info);
   GL(dl_rtld_map).l_mach = info->l.l_mach;
@@ -546,7 +547,7 @@ _dl_start (void *arg)
 
   /* Read our own dynamic section and fill in the info array.  */
   bootstrap_map.l_ld = (void *) bootstrap_map.l_addr + elf_machine_dynamic ();
-  elf_get_dynamic_info (&bootstrap_map, NULL);
+  elf_get_dynamic_info (&bootstrap_map);
 
 #if NO_TLS_OFFSET != 0
   bootstrap_map.l_tls_offset = NO_TLS_OFFSET;
@@ -1468,6 +1469,7 @@ dl_main (const ElfW(Phdr) *phdr,
 	/* This tells us where to find the dynamic section,
 	   which tells us everything we need to do.  */
 	main_map->l_ld = (void *) main_map->l_addr + ph->p_vaddr;
+	main_map->l_ld_readonly = (ph->p_flags & PF_W) == 0;
 	break;
       case PT_INTERP:
 	/* This "interpreter segment" was used by the program loader to
@@ -1613,7 +1615,7 @@ dl_main (const ElfW(Phdr) *phdr,
   if (! rtld_is_main)
     {
       /* Extract the contents of the dynamic section for easy access.  */
-      elf_get_dynamic_info (main_map, NULL);
+      elf_get_dynamic_info (main_map);
 
       /* If the main map is libc.so, update the base namespace to
 	 refer to this map.  If libc.so is loaded later, this happens

elf/setup-vdso.h

@@ -33,8 +33,6 @@ setup_vdso (struct link_map *main_map __attribute__ ((unused)),
 				       0, LM_ID_BASE);
   if (__glibc_likely (l != NULL))
     {
-      static ElfW(Dyn) dyn_temp[DL_RO_DYN_TEMP_CNT] attribute_relro;
-
       l->l_phdr = ((const void *) GLRO(dl_sysinfo_dso)
 		   + GLRO(dl_sysinfo_dso)->e_phoff);
       l->l_phnum = GLRO(dl_sysinfo_dso)->e_phnum;
@@ -45,6 +43,7 @@ setup_vdso (struct link_map *main_map __attribute__ ((unused)),
 	    {
 	      l->l_ld = (void *) ph->p_vaddr;
 	      l->l_ldnum = ph->p_memsz / sizeof (ElfW(Dyn));
+	      l->l_ld_readonly = (ph->p_flags & PF_W) == 0;
 	    }
 	  else if (ph->p_type == PT_LOAD)
 	    {
@@ -65,7 +64,7 @@ setup_vdso (struct link_map *main_map __attribute__ ((unused)),
       l->l_map_end += l->l_addr;
       l->l_text_end += l->l_addr;
       l->l_ld = (void *) ((ElfW(Addr)) l->l_ld + l->l_addr);
-      elf_get_dynamic_info (l, dyn_temp);
+      elf_get_dynamic_info (l);
       _dl_setup_hash (l);
       l->l_relocated = 1;
 

elf/tst-ro-dynamic-mod.c

@@ -0,0 +1,19 @@
+/* Test case for DSO with readonly dynamic section.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+int foo = -1;

elf/tst-ro-dynamic-mod.map

@@ -0,0 +1,16 @@
+SECTIONS
+{
+ . = SIZEOF_HEADERS;
+ .dynamic : { *(.dynamic) } :text :dynamic
+ .rodata : { *(.data*) *(.bss*) } :text
+ /DISCARD/ : {
+  *(.note.gnu.property)
+ }
+ .note : { *(.note.*) } :text :note
+}
+PHDRS
+{
+ text PT_LOAD FLAGS(5) FILEHDR PHDRS;
+ dynamic PT_DYNAMIC FLAGS(4);
+ note PT_NOTE FLAGS(4);
+}

elf/tst-ro-dynamic.c

@@ -0,0 +1,31 @@
+/* Test case for DSO with readonly dynamic section.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/check.h>
+#include <support/test-driver.h>
+
+extern int foo;
+
+static int
+do_test (void)
+{
+  TEST_COMPARE (foo, -1);
+  return 0;
+}
+
+#include <support/test-driver.c>

include/link.h

@@ -205,6 +205,7 @@ struct link_map
     unsigned int l_free_initfini:1; /* Nonzero if l_initfini can be
 				       freed, ie. not allocated with
 				       the dummy malloc in ld.so.  */
+    unsigned int l_ld_readonly:1; /* Nonzero if dynamic section is readonly.  */
 
     /* NODELETE status of the map.  Only valid for maps of type
        lt_loaded.  Lazy binding sets l_nodelete_active directly,
@@ -342,6 +343,8 @@ struct link_map
     unsigned long long int l_serial;
   };
 
+#include <dl-relocate-ld.h>
+
 /* Information used by audit modules.  For most link maps, this data
    immediate follows the link map in memory.  For the dynamic linker,
    it is allocated separately.  See link_map_audit_state in

sysdeps/generic/dl-relocate-ld.h

@@ -0,0 +1,32 @@
+/* Check if dynamic section should be relocated.  Generic version.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _DL_RELOCATE_LD_H
+#define _DL_RELOCATE_LD_H
+
+/* Return true if dynamic section in the shared library L should be
+   relocated.  */
+
+static inline bool
+dl_relocate_ld (const struct link_map *l)
+{
+  /* Don't relocate dynamic section if it is readonly  */
+  return !l->l_ld_readonly;
+}
+
+#endif /* _DL_RELOCATE_LD_H */

sysdeps/generic/ldsodefs.h

@@ -75,11 +75,8 @@ __BEGIN_DECLS
   have to be accessed via the D_PTR macro.  The macro is needed since for
   most architectures the entry is already relocated - but for some not
   and we need to relocate at access time.  */
-#ifdef DL_RO_DYN_SECTION
-# define D_PTR(map, i) ((map)->i->d_un.d_ptr + (map)->l_addr)
-#else
-# define D_PTR(map, i) (map)->i->d_un.d_ptr
-#endif
+#define D_PTR(map, i) \
+  ((map)->i->d_un.d_ptr + (dl_relocate_ld (map) ? 0 : (map)->l_addr))
 
 /* Result of the lookup functions and how to retrieve the base address.  */
 typedef struct link_map *lookup_t;

sysdeps/mips/dl-relocate-ld.h

@@ -0,0 +1,32 @@
+/* Check if dynamic section should be relocated.  MIPS version.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _DL_RELOCATE_LD_H
+#define _DL_RELOCATE_LD_H
+
+/* Return true if dynamic section in the shared library L should be
+   relocated.  */
+
+static inline bool
+dl_relocate_ld (const struct link_map *l)
+{
+  /* Never relocate dynamic section.  */
+  return false;
+}
+
+#endif /* _DL_RELOCATE_LD_H */

sysdeps/mips/ldsodefs.h

@@ -75,10 +75,6 @@ struct La_mips_64_retval;
 					  struct La_mips_64_retval *,	    \
 					  const char *);
 
-/* The MIPS ABI specifies that the dynamic section has to be read-only.  */
-
-#define DL_RO_DYN_SECTION 1
-
 #include_next <ldsodefs.h>
 
 /* The 64-bit MIPS ELF ABI uses an unusual reloc format.  Each

sysdeps/riscv/dl-relocate-ld.h

@@ -0,0 +1,32 @@
+/* Check if dynamic section should be relocated.  RISC-V version.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _DL_RELOCATE_LD_H
+#define _DL_RELOCATE_LD_H
+
+/* Return true if dynamic section in the shared library L should be
+   relocated.  */
+
+static inline bool
+dl_relocate_ld (const struct link_map *l)
+{
+  /* Never relocate dynamic section for ABI compatibility.  */
+  return false;
+}
+
+#endif /* _DL_RELOCATE_LD_H */

sysdeps/riscv/ldsodefs.h

@@ -38,11 +38,6 @@ struct La_riscv_retval;
 				       struct La_riscv_retval *,	\
 				       const char *);
 
-/* Although the RISC-V ABI does not specify that the dynamic section has
-   to be read-only, it needs to be kept for ABI compatibility.  */
-
-#define DL_RO_DYN_SECTION 1
-
 #include_next <ldsodefs.h>
 
 #endif