Re: [PATCH v5 1/5] elf: Support DT_RELR relative relocation format [BZ #27924]

[Fangrui Song <maskray@google.com>]

On 2022-03-02, H.J. Lu wrote:
>From: Fangrui Song <maskray@google.com>
>
>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
>
>Changes from the original patch:
>
>1. Check the linker option, -z pack-relative-relocs, which add a
>GLIBC_ABI_DT_RELR symbol version dependency on the shared C library if
>it provides a GLIBC_2.XX symbol version.
>2. Change make variale to have-dt-relr.
>3. Rename tst-relr-no-pie to tst-relr-pie for --disable-default-pie.
>4. Use TEST_VERIFY in tst-relr.c.
>5. Add the check-tst-relr-pie.out test to check for linker generated
>libc.so version dependency on GLIBC_ABI_DT_RELR.
>---
> configure              | 42 +++++++++++++++++++++++++++
> configure.ac           | 10 +++++++
> elf/Makefile           | 14 +++++++++
> elf/dynamic-link.h     | 34 ++++++++++++++++++++++
> elf/elf.h              | 13 +++++++--
> elf/get-dynamic-info.h |  3 ++
> elf/tst-relr-pie.c     |  1 +
> elf/tst-relr.c         | 64 ++++++++++++++++++++++++++++++++++++++++++
> 8 files changed, 179 insertions(+), 2 deletions(-)
> create mode 100644 elf/tst-relr-pie.c
> create mode 100644 elf/tst-relr.c
>
>diff --git a/configure b/configure
>index 8e5bee775a..9156e29fe9 100755
>--- a/configure
>+++ b/configure
>@@ -6115,6 +6115,48 @@ $as_echo "$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 -z pack-relative-relocs" >&5
>+$as_echo_n "checking for linker that supports -z pack-relative-relocs... " >&6; }
>+libc_linker_feature=no
>+if test x"$gnu_ld" = x"yes"; then
>+  cat > conftest.c <<EOF
>+int _start (void) { return 42; }
>+EOF
>+  if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
>+		    -Wl,-z,pack-relative-relocs -nostdlib -nostartfiles
>+		    -fPIC -shared -o conftest.so conftest.c
>+		    1>&5'
>+  { { eval echo "\"\$as_me\":${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
>+    if ${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp -Wl,-z,pack-relative-relocs -nostdlib \
>+	-nostartfiles -fPIC -shared -o conftest.so conftest.c 2>&1 \
>+	| grep "warning: -z pack-relative-relocs ignored" > /dev/null 2>&1; then
>+      true
>+    else
>+      libc_linker_feature=yes
>+    fi
>+  fi
>+  rm -f conftest*
>+fi
>+if test $libc_linker_feature = yes; then
>+  libc_cv_dt_relr=yes
>+else
>+  libc_cv_dt_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-dt-relr = $libc_cv_dt_relr"
>+if test "$libc_cv_dt_relr" = yes; then
>+  have_dt_relr=1
>+else
>+  have_dt_relr=0
>+fi
>+
> { $as_echo "$as_me:${as_lineno-$LINENO}: checking for linker that supports --no-dynamic-linker" >&5
> $as_echo_n "checking for linker that supports --no-dynamic-linker... " >&6; }
> libc_linker_feature=no
>diff --git a/configure.ac b/configure.ac
>index 87f67d25ec..5c09871fee 100644
>--- a/configure.ac
>+++ b/configure.ac
>@@ -1367,6 +1367,16 @@ LIBC_LINKER_FEATURE([--depaudit], [-Wl,--depaudit,x],
> 		    [libc_cv_depaudit=yes], [libc_cv_depaudit=no])
> LIBC_CONFIG_VAR([have-depaudit], [$libc_cv_depaudit])
>
>+LIBC_LINKER_FEATURE([-z pack-relative-relocs],
>+		    [-Wl,-z,pack-relative-relocs],
>+		    [libc_cv_dt_relr=yes], [libc_cv_dt_relr=no])
>+LIBC_CONFIG_VAR([have-dt-relr], [$libc_cv_dt_relr])
>+if test "$libc_cv_dt_relr" = yes; then
>+  have_dt_relr=1
>+else
>+  have_dt_relr=0
>+fi
>+
> 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 c96924e9c2..fd462ba315 100644
>--- a/elf/Makefile
>+++ b/elf/Makefile
>@@ -541,6 +541,14 @@ tests-special += \
>   # tests-special
> endif
> endif
>+ifeq ($(have-dt-relr),yes)
>+tests += tst-relr tst-relr-pie
>+tests-pie += tst-relr-pie
>+tests-special += $(objpfx)check-tst-relr-pie.out
>+CFLAGS-tst-relr-pie.c += $(pie-ccflag)
>+LDFLAGS-tst-relr += -Wl,-z,pack-relative-relocs
>+LDFLAGS-tst-relr-pie += -Wl,-z,pack-relative-relocs
>+endif
> endif
>
> ifeq ($(run-built-tests),yes)
>@@ -2725,3 +2733,9 @@ $(objpfx)tst-p_align3: $(objpfx)tst-p_alignmod3.so
> $(objpfx)tst-p_align3.out: tst-p_align3.sh $(objpfx)tst-p_align3
> 	$(SHELL) $< $(common-objpfx) '$(test-program-prefix)'; \
> 	$(evaluate-test)
>+
>+$(objpfx)check-tst-relr-pie.out: $(objpfx)tst-relr-pie
>+	LC_ALL=C $(OBJDUMP) -p $< \
>+		| sed -ne '/required from libc.so/,$$ p' \
>+		| grep GLIBC_ABI_DT_RELR > $@; \
>+	$(evaluate-test)
>diff --git a/elf/dynamic-link.h b/elf/dynamic-link.h
>index 25dd7ca4f2..5318079bac 100644
>--- a/elf/dynamic-link.h
>+++ b/elf/dynamic-link.h
>@@ -146,14 +146,48 @@ 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.  */
>+# ifdef RTLD_BOOTSTRAP
>+#  define DO_RTLD_BOOTSTRAP 1
>+# else
>+#  define DO_RTLD_BOOTSTRAP 0
>+# endif
> # define ELF_DYNAMIC_RELOCATE(map, scope, lazy, consider_profile, skip_ifunc) \
>   do {									      \
>     int edr_lazy = elf_machine_runtime_setup ((map), (scope), (lazy),	      \
> 					      (consider_profile));	      \
>     ELF_DYNAMIC_DO_REL ((map), (scope), edr_lazy, skip_ifunc);		      \
>     ELF_DYNAMIC_DO_RELA ((map), (scope), edr_lazy, skip_ifunc);		      \
>+    if (((map) != &GL(dl_rtld_map) || DO_RTLD_BOOTSTRAP))		      \
>+      ELF_DYNAMIC_DO_RELR (map);					      \
>   } while (0)
>
> #endif

As mentioned on a binutils thread, ELF_DYNAMIC_DO_RELR needs to be
reordered before DL_REL/DO_RELA to help powerpc get the support in the future
https://sourceware.org/git/?p=glibc.git;a=blobdiff;f=elf/dynamic-link.h;h=169745f9a409aec7995f7418736ae64bfeebc9d6;hp=25dd7ca4f2eca3c97fdd1649c15e1b5106e70dc1;hb=1c6cc29baf9a4c7129ab2e94b0d4022bfa4f3299;hpb=edc696a73a7cb07b1aa68792a845a98d036ee7eb

>diff --git a/elf/elf.h b/elf/elf.h
>index 0735f6b579..0195029188 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 6c2ccd6db4..6c2a3a12b1 100644
>--- a/elf/get-dynamic-info.h
>+++ b/elf/get-dynamic-info.h
>@@ -89,6 +89,7 @@ elf_get_dynamic_info (struct link_map *l, bool bootstrap,
> # 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));
>@@ -113,6 +114,8 @@ elf_get_dynamic_info (struct link_map *l, bool bootstrap,
>   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)));
>   if (bootstrap || static_pie_bootstrap)
>     {
>       assert (info[DT_RUNPATH] == NULL);
>diff --git a/elf/tst-relr-pie.c b/elf/tst-relr-pie.c
>new file mode 100644
>index 0000000000..7df0cdbfd6
>--- /dev/null
>+++ b/elf/tst-relr-pie.c
>@@ -0,0 +1 @@
>+#include "tst-relr.c"
>diff --git a/elf/tst-relr.c b/elf/tst-relr.c
>new file mode 100644
>index 0000000000..56addd2ad4
>--- /dev/null
>+++ b/elf/tst-relr.c
>@@ -0,0 +1,64 @@
>+/* Basic tests for DT_RELR.
>+   Copyright (C) 2022 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 <link.h>
>+#include <stdbool.h>
>+#include <support/check.h>
>+
>+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)
>+{
>+  ElfW(Dyn) *d = _DYNAMIC;
>+  if (d)
>+    {
>+      bool has_relr = false;
>+      for (; d->d_tag != DT_NULL; d++)
>+	if (d->d_tag == DT_RELR)
>+	  has_relr = true;
>+
>+#if defined __PIE__ || defined __pie__ || defined PIE || defined pie
>+      TEST_VERIFY (has_relr);
>+#else
>+      TEST_VERIFY (!has_relr);
>+#endif
>+    }
>+
>+  for (int i = 0; i < sizeof (arr) / sizeof (arr[0]); i++)
>+    TEST_VERIFY ((arr[i] == 0 && val[i] == 0)
>+		 || (arr[i] == &o && val[i] == 1)
>+		 || (arr[i] == &x && val[i] == 2));
>+
>+  return 0;
>+}
>+
>+#include <support/test-driver.c>
>-- 
>2.35.1
>