[PATCH v2] elf: Support DT_RELR relative relocation format [BZ #27924]

Fangrui Song maskray@google.com
Sun Oct 17 00:50:20 GMT 2021


PIE and shared objects usually have many relative relocations. In
2017/2018, SHT_RELR/DT_RELR was proposed on
https://groups.google.com/g/generic-abi/c/bX460iggiKg/m/GxjM0L-PBAAJ
("Proposal for a new section type SHT_RELR") and is a pre-standard. RELR
usually takes 3% or smaller space than R_*_RELATIVE relocations. The
virtual memory size of a mostly statically linked PIE is typically 5~10%
smaller.

---

Notes I will not include in the submitted commit:

Available on https://sourceware.org/git/?p=glibc.git;a=shortlog;h=refs/heads/maskray/relr

"pre-standard": even Solaris folks are happy with the refined generic-abi
proposal. Cary Coutant will apply the change
https://sourceware.org/pipermail/libc-alpha/2021-October/131781.html

This patch is simpler than Chrome OS's glibc patch and makes ELF_DYNAMIC_DO_RELR
available to all ports. I don't think the current glibc implementation
supports ia64 in an ELFCLASS32 container. That said, the style I used is
works with an ELFCLASS32 container for 64-bit machine if ElfW(Addr) is
64-bit.

* Chrome OS folks have carried a local patch since 2018 (latest version:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/refs/heads/main/sys-libs/glibc/files/local/glibc-2.32).
  I.e. this feature has been battle tested.
* Android bionic supports 2018 and switched to DT_RELR==36 in 2020.
* The Linux kernel has supported CONFIG_RELR since 2019-08
  (https://git.kernel.org/linus/5cf896fb6be3effd9aea455b22213e27be8bdb1d).
* A musl patch (by me) exists but is not applied:
  https://www.openwall.com/lists/musl/2019/03/06/3
* rtld-elf from FreeBSD 14 will support DT_RELR.

I believe upstream glibc should support DT_RELR to benefit all Linux
distributions. I filed some feature requests to get their attention:

* Gentoo: https://bugs.gentoo.org/818376
* Arch Linux: https://bugs.archlinux.org/task/72433
* Debian https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=996598
* Fedora https://bugzilla.redhat.com/show_bug.cgi?id=2014699

As of linker support (to the best of my knowledge):

* LLD support DT_RELR.
* https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/refs/heads/main/sys-devel/binutils/files/
  has a gold patch.
* GNU ld feature request https://sourceware.org/bugzilla/show_bug.cgi?id=27923

I wish that GNU ld and gold maintainers can implement the feature as well :)

Tested on aarch64 and x86_64.

Changes from v1 (https://sourceware.org/pipermail/libc-alpha/2021-October/131768.html)
* Fix style, simplify code
* Improve test
---
 configure              | 31 +++++++++++++++++++++++++++++++
 configure.ac           |  4 ++++
 elf/Makefile           |  4 ++++
 elf/dynamic-link.h     | 28 ++++++++++++++++++++++++++++
 elf/elf.h              | 13 +++++++++++--
 elf/get-dynamic-info.h |  3 +++
 elf/tst-relr.c         | 27 +++++++++++++++++++++++++++
 7 files changed, 108 insertions(+), 2 deletions(-)
 create mode 100644 elf/tst-relr.c

diff --git a/configure b/configure
index 3227e434d3..fdab6a97ef 100755
--- a/configure
+++ b/configure
@@ -6067,6 +6067,37 @@ $as_echo "$libc_linker_feature" >&6; }
 config_vars="$config_vars
 have-depaudit = $libc_cv_depaudit"
 
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports --pack-dyn-relocs=relr" >&5
+$as_echo_n "checking for linker that supports --pack-dyn-relocs=relr... " >&6; }
+libc_linker_feature=no
+if test x"$gnu_ld" = x"yes"; then
+  cat > conftest.c <<EOF
+int _start (void) { return 42; }
+EOF
+  if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
+		    -Wl,--pack-dyn-relocs=relr -nostdlib -nostartfiles
+		    -fPIC -shared -o conftest.so conftest.c
+		    1>&5'
+  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
+  (eval $ac_try) 2>&5
+  ac_status=$?
+  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; }; }
+  then
+    libc_linker_feature=yes
+  fi
+  rm -f conftest*
+fi
+if test $libc_linker_feature = yes; then
+  libc_cv_relr=yes
+else
+  libc_cv_relr=no
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_linker_feature" >&5
+$as_echo "$libc_linker_feature" >&6; }
+config_vars="$config_vars
+have-relr = $libc_cv_relr"
+
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports --no-dynamic-linker" >&5
 $as_echo_n "checking for linker that supports --no-dynamic-linker... " >&6; }
 libc_linker_feature=no
diff --git a/configure.ac b/configure.ac
index 00f49f09f7..96110f9d7d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1354,6 +1354,10 @@ LIBC_LINKER_FEATURE([--depaudit], [-Wl,--depaudit,x],
 		    [libc_cv_depaudit=yes], [libc_cv_depaudit=no])
 LIBC_CONFIG_VAR([have-depaudit], [$libc_cv_depaudit])
 
+LIBC_LINKER_FEATURE([--pack-dyn-relocs=relr], [-Wl,--pack-dyn-relocs=relr],
+		    [libc_cv_relr=yes], [libc_cv_relr=no])
+LIBC_CONFIG_VAR([have-relr], [$libc_cv_relr])
+
 LIBC_LINKER_FEATURE([--no-dynamic-linker],
 		    [-Wl,--no-dynamic-linker],
 		    [libc_cv_no_dynamic_linker=yes],
diff --git a/elf/Makefile b/elf/Makefile
index bf45d8ee24..2c4cdfac68 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -245,6 +245,10 @@ tests-special += $(objpfx)tst-audit14-cmp.out $(objpfx)tst-audit15-cmp.out \
 		 $(objpfx)tst-audit16-cmp.out
 endif
 endif
+ifeq ($(have-relr),yes)
+tests += tst-relr
+LDFLAGS-tst-relr += -Wl,--pack-dyn-relocs=relr
+endif
 endif
 tests += $(tests-execstack-$(have-z-execstack))
 ifeq ($(run-built-tests),yes)
diff --git a/elf/dynamic-link.h b/elf/dynamic-link.h
index ac4cc70dea..fafd42f918 100644
--- a/elf/dynamic-link.h
+++ b/elf/dynamic-link.h
@@ -160,6 +160,33 @@ elf_machine_lazy_rel (struct link_map *map, struct r_scope_elem *scope[],
 #  define ELF_DYNAMIC_DO_RELA(map, scope, lazy, skip_ifunc) /* Nothing to do.  */
 # endif
 
+# define ELF_DYNAMIC_DO_RELR(map)					      \
+  do {									      \
+    ElfW(Addr) l_addr = (map)->l_addr, *where = 0;			      \
+    const ElfW(Relr) *r, *end;						      \
+    if (!(map)->l_info[DT_RELR])					      \
+      break;								      \
+    r = (const ElfW(Relr) *)D_PTR((map), l_info[DT_RELR]);		      \
+    end = (const ElfW(Relr) *)((const char *)r +			      \
+                               (map)->l_info[DT_RELRSZ]->d_un.d_val);	      \
+    for (; r < end; r++)						      \
+      {									      \
+	ElfW(Relr) entry = *r;						      \
+	if ((entry & 1) == 0)						      \
+	  {								      \
+	    where = (ElfW(Addr) *)(l_addr + entry);			      \
+	    *where++ += l_addr;						      \
+	  }								      \
+	else 								      \
+	  {								      \
+	    for (long i = 0; (entry >>= 1) != 0; i++)			      \
+	      if ((entry & 1) != 0)					      \
+		where[i] += l_addr;					      \
+	    where += CHAR_BIT * sizeof(ElfW(Relr)) - 1;			      \
+	  }								      \
+      }									      \
+  } while (0);
+
 /* This can't just be an inline function because GCC is too dumb
    to inline functions containing inlines themselves.  */
 # define ELF_DYNAMIC_RELOCATE(map, scope, lazy, consider_profile, skip_ifunc) \
@@ -168,6 +195,7 @@ elf_machine_lazy_rel (struct link_map *map, struct r_scope_elem *scope[],
 					      (consider_profile));	      \
     ELF_DYNAMIC_DO_REL ((map), (scope), edr_lazy, skip_ifunc);		      \
     ELF_DYNAMIC_DO_RELA ((map), (scope), edr_lazy, skip_ifunc);		      \
+    ELF_DYNAMIC_DO_RELR ((map));					      \
   } while (0)
 
 #endif
diff --git a/elf/elf.h b/elf/elf.h
index 50f87baceb..6dc1ee0ac0 100644
--- a/elf/elf.h
+++ b/elf/elf.h
@@ -443,7 +443,8 @@ typedef struct
 #define SHT_PREINIT_ARRAY 16		/* Array of pre-constructors */
 #define SHT_GROUP	  17		/* Section group */
 #define SHT_SYMTAB_SHNDX  18		/* Extended section indices */
-#define	SHT_NUM		  19		/* Number of defined types.  */
+#define SHT_RELR	  19            /* RELR relative relocations */
+#define	SHT_NUM		  20		/* Number of defined types.  */
 #define SHT_LOOS	  0x60000000	/* Start OS-specific.  */
 #define SHT_GNU_ATTRIBUTES 0x6ffffff5	/* Object attributes.  */
 #define SHT_GNU_HASH	  0x6ffffff6	/* GNU-style hash table.  */
@@ -662,6 +663,11 @@ typedef struct
   Elf64_Sxword	r_addend;		/* Addend */
 } Elf64_Rela;
 
+/* RELR relocation table entry */
+
+typedef Elf32_Word	Elf32_Relr;
+typedef Elf64_Xword	Elf64_Relr;
+
 /* How to extract and insert information held in the r_info field.  */
 
 #define ELF32_R_SYM(val)		((val) >> 8)
@@ -887,7 +893,10 @@ typedef struct
 #define DT_PREINIT_ARRAY 32		/* Array with addresses of preinit fct*/
 #define DT_PREINIT_ARRAYSZ 33		/* size in bytes of DT_PREINIT_ARRAY */
 #define DT_SYMTAB_SHNDX	34		/* Address of SYMTAB_SHNDX section */
-#define	DT_NUM		35		/* Number used */
+#define DT_RELRSZ	35		/* Total size of RELR relative relocations */
+#define DT_RELR		36		/* Address of RELR relative relocations */
+#define DT_RELRENT	37		/* Size of one RELR relative relocaction */
+#define	DT_NUM		38		/* Number used */
 #define DT_LOOS		0x6000000d	/* Start of OS-specific */
 #define DT_HIOS		0x6ffff000	/* End of OS-specific */
 #define DT_LOPROC	0x70000000	/* Start of processor-specific */
diff --git a/elf/get-dynamic-info.h b/elf/get-dynamic-info.h
index 1ac0663d1f..ce0151c2b4 100644
--- a/elf/get-dynamic-info.h
+++ b/elf/get-dynamic-info.h
@@ -88,6 +88,7 @@ elf_get_dynamic_info (struct link_map *l)
 # if ! ELF_MACHINE_NO_REL
       ADJUST_DYN_INFO (DT_REL);
 # endif
+      ADJUST_DYN_INFO (DT_RELR);
       ADJUST_DYN_INFO (DT_JMPREL);
       ADJUST_DYN_INFO (VERSYMIDX (DT_VERSYM));
       ADJUST_DYN_INFO (ADDRIDX (DT_GNU_HASH));
@@ -112,6 +113,8 @@ elf_get_dynamic_info (struct link_map *l)
   if (info[DT_REL] != NULL)
     assert (info[DT_RELENT]->d_un.d_val == sizeof (ElfW(Rel)));
 #endif
+  if (info[DT_RELR] != NULL)
+    assert (info[DT_RELRENT]->d_un.d_val == sizeof (ElfW(Relr)));
 #ifdef STATIC_PIE_BOOTSTRAP
   assert (info[DT_RUNPATH] == NULL);
   assert (info[DT_RPATH] == NULL);
diff --git a/elf/tst-relr.c b/elf/tst-relr.c
new file mode 100644
index 0000000000..79736a4a8a
--- /dev/null
+++ b/elf/tst-relr.c
@@ -0,0 +1,27 @@
+static int o, x;
+
+#define ELEMS O O O O O O O O X X X X X X X O O X O O X X X E X E E O X O E
+#define E 0,
+
+#define O &o,
+#define X &x,
+void *arr[] = { ELEMS };
+#undef O
+#undef X
+
+#define O 1,
+#define X 2,
+static char val[] = { ELEMS };
+
+static int
+do_test (void)
+{
+  for (int i = 0; i < sizeof (arr) / sizeof (arr[0]); i++)
+    if (!((arr[i] == 0 && val[i] == 0) ||
+	  (arr[i] == &o && val[i] == 1) ||
+	  (arr[i] == &x && val[i] == 2)))
+      return 1;
+  return 0;
+}
+
+#include <support/test-driver.c>
-- 
2.33.0.1079.g6e70778dc9-goog



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