[PATCH] elf: Support DT_RELR relative relocation format [BZ #27924]
Fangrui Song
maskray@google.com
Fri Oct 8 06:57:40 GMT 2021
PIC objects (especially PIE and symbolic 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 welcomed by many parties
(including Solaris). This packed format can typically save 95% dynamic
relocation section size for PIE. The vaddr size of a PIE can be 10% smaller.
* Chrome OS folks have carried a local patch for a while (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 DT_RELR.
* 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
I believe upstream glibc should support DT_RELR to benefit all Linux
distributions.
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 :)
This patch is simpler than Chrome OS's glibc patch and makes ELF_DYNAMIC_DO_RELR
available to all ports.
I have adjusted aclocal.m4, otherwise it thinks ld.lld doesn't support
--pack-dyn-relocs=relr just because ld.lld -v --help doesn't contain the literal
string.
% ld.lld -v --help | grep pack-dyn-relocs
--pack-dyn-relocs=[none,android,relr,android+relr]
(`$gnu_ld` is a lie: both gold and ld.lld's "User-Agent:" strings contain
"GNU" and therefore make gnu_ld=true.)
Tested on aarch64 and x86_64.
---
aclocal.m4 | 19 +++-----
configure | 107 ++++++++++++++++++++++++-----------------
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 | 20 ++++++++
8 files changed, 141 insertions(+), 57 deletions(-)
create mode 100644 elf/tst-relr.c
diff --git a/aclocal.m4 b/aclocal.m4
index c195c4db56..65a12df047 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -224,20 +224,17 @@ AC_DEFUN([LIBC_LINKER_FEATURE],
[AC_MSG_CHECKING([for linker that supports $1])
libc_linker_feature=no
if test x"$gnu_ld" = x"yes"; then
- libc_linker_check=`$LD -v --help 2>/dev/null | grep "\$1"`
- if test -n "$libc_linker_check"; then
- cat > conftest.c <<EOF
+ cat > conftest.c <<EOF
int _start (void) { return 42; }
EOF
- if AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
- $2 -nostdlib -nostartfiles
- -fPIC -shared -o conftest.so conftest.c
- 1>&AS_MESSAGE_LOG_FD])
- then
- libc_linker_feature=yes
- fi
- rm -f conftest*
+ if AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
+ $2 -nostdlib -nostartfiles
+ -fPIC -shared -o conftest.so conftest.c
+ 1>&AS_MESSAGE_LOG_FD])
+ then
+ libc_linker_feature=yes
fi
+ rm -f conftest*
fi
if test $libc_linker_feature = yes; then
$3
diff --git a/configure b/configure
index 39d75eb4ed..fdab6a97ef 100755
--- a/configure
+++ b/configure
@@ -5979,25 +5979,22 @@ fi
$as_echo_n "checking for linker that supports -z execstack... " >&6; }
libc_linker_feature=no
if test x"$gnu_ld" = x"yes"; then
- libc_linker_check=`$LD -v --help 2>/dev/null | grep "\-z execstack"`
- if test -n "$libc_linker_check"; then
- cat > conftest.c <<EOF
+ cat > conftest.c <<EOF
int _start (void) { return 42; }
EOF
- if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
- -Wl,-z,execstack -nostdlib -nostartfiles
- -fPIC -shared -o conftest.so conftest.c
- 1>&5'
+ if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
+ -Wl,-z,execstack -nostdlib -nostartfiles
+ -fPIC -shared -o conftest.so conftest.c
+ 1>&5'
{ { eval echo "\"\$as_me\":${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*
+ then
+ libc_linker_feature=yes
fi
+ rm -f conftest*
fi
if test $libc_linker_feature = yes; then
libc_cv_z_execstack=yes
@@ -6012,25 +6009,22 @@ $as_echo "$libc_linker_feature" >&6; }
$as_echo_n "checking for linker that supports -z start-stop-gc... " >&6; }
libc_linker_feature=no
if test x"$gnu_ld" = x"yes"; then
- libc_linker_check=`$LD -v --help 2>/dev/null | grep "\-z start-stop-gc"`
- if test -n "$libc_linker_check"; then
- cat > conftest.c <<EOF
+ cat > conftest.c <<EOF
int _start (void) { return 42; }
EOF
- if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
- -Wl,-z,start-stop-gc -nostdlib -nostartfiles
- -fPIC -shared -o conftest.so conftest.c
- 1>&5'
+ if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
+ -Wl,-z,start-stop-gc -nostdlib -nostartfiles
+ -fPIC -shared -o conftest.so conftest.c
+ 1>&5'
{ { eval echo "\"\$as_me\":${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*
+ then
+ libc_linker_feature=yes
fi
+ rm -f conftest*
fi
if test $libc_linker_feature = yes; then
libc_cv_z_start_stop_gc=yes
@@ -6046,25 +6040,22 @@ have-z-start-stop-gc = $libc_cv_z_start_stop_gc"
$as_echo_n "checking for linker that supports --depaudit... " >&6; }
libc_linker_feature=no
if test x"$gnu_ld" = x"yes"; then
- libc_linker_check=`$LD -v --help 2>/dev/null | grep "\--depaudit"`
- if test -n "$libc_linker_check"; then
- cat > conftest.c <<EOF
+ cat > conftest.c <<EOF
int _start (void) { return 42; }
EOF
- if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
- -Wl,--depaudit,x -nostdlib -nostartfiles
- -fPIC -shared -o conftest.so conftest.c
- 1>&5'
+ if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
+ -Wl,--depaudit,x -nostdlib -nostartfiles
+ -fPIC -shared -o conftest.so conftest.c
+ 1>&5'
{ { eval echo "\"\$as_me\":${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*
+ then
+ libc_linker_feature=yes
fi
+ rm -f conftest*
fi
if test $libc_linker_feature = yes; then
libc_cv_depaudit=yes
@@ -6076,29 +6067,57 @@ $as_echo "$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
$as_echo_n "checking for linker that supports --no-dynamic-linker... " >&6; }
libc_linker_feature=no
if test x"$gnu_ld" = x"yes"; then
- libc_linker_check=`$LD -v --help 2>/dev/null | grep "\--no-dynamic-linker"`
- if test -n "$libc_linker_check"; then
- cat > conftest.c <<EOF
+ cat > conftest.c <<EOF
int _start (void) { return 42; }
EOF
- if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
- -Wl,--no-dynamic-linker -nostdlib -nostartfiles
- -fPIC -shared -o conftest.so conftest.c
- 1>&5'
+ if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
+ -Wl,--no-dynamic-linker -nostdlib -nostartfiles
+ -fPIC -shared -o conftest.so conftest.c
+ 1>&5'
{ { eval echo "\"\$as_me\":${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*
+ then
+ libc_linker_feature=yes
fi
+ rm -f conftest*
fi
if test $libc_linker_feature = yes; then
libc_cv_no_dynamic_linker=yes
diff --git a/configure.ac b/configure.ac
index 00f49f09f7..96110f9d7d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1354,6 +1354,10 @@ LIBC_LINKER_FEATURE([--depaudit], [-Wl,--depaudit,x],
[libc_cv_depaudit=yes], [libc_cv_depaudit=no])
LIBC_CONFIG_VAR([have-depaudit], [$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], [$libc_cv_relr])
+
LIBC_LINKER_FEATURE([--no-dynamic-linker],
[-Wl,--no-dynamic-linker],
[libc_cv_no_dynamic_linker=yes],
diff --git a/elf/Makefile b/elf/Makefile
index 26986c0692..e7c3f3404b 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -241,6 +241,10 @@ endif
ifeq ($(have-depaudit),yes)
tests += tst-audit14 tst-audit15 tst-audit16
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 7cc3021164..ce8d628ca1 100644
--- a/elf/dynamic-link.h
+++ b/elf/dynamic-link.h
@@ -192,6 +192,33 @@ 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, base = 0, start; \
+ const ElfW(Relr) *r = 0, *end = 0; \
+ if (!(map)->l_info[DT_RELR]) \
+ break; \
+ start = D_PTR((map), l_info[DT_RELR]); \
+ r = (const ElfW(Relr) *)start; \
+ end = (const ElfW(Relr) *)(start + (map)->l_info[DT_RELRSZ]->d_un.d_val); \
+ for (; r < end; ++r) { \
+ ElfW(Relr) entry = *r; \
+ if ((entry & 1) == 0) { \
+ *((ElfW(Addr) *)(l_addr + entry)) += l_addr; \
+ base = entry + sizeof(ElfW(Addr)); \
+ continue; \
+ } \
+ ElfW(Addr) offset = base; \
+ do { \
+ entry >>= 1; \
+ if ((entry & 1) != 0) \
+ *((ElfW(Addr) *)(l_addr + offset)) += l_addr; \
+ offset += sizeof(ElfW(Addr)); \
+ } while (entry != 0); \
+ base += (8 * sizeof(ElfW(Relr)) - 1) * sizeof(ElfW(Addr)); \
+ } \
+ } 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) \
@@ -200,6 +227,7 @@ 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..093d69fd07 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
+#define DT_RELR 36
+#define DT_RELRENT 37
+#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 15c316b38c..08e3aae68b 100644
--- a/elf/get-dynamic-info.h
+++ b/elf/get-dynamic-info.h
@@ -87,6 +87,7 @@ 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));
@@ -111,6 +112,8 @@ 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 RTLD_BOOTSTRAP
/* Only the bind now flags are allowed. */
assert (info[VERSYMIDX (DT_FLAGS_1)] == NULL
diff --git a/elf/tst-relr.c b/elf/tst-relr.c
new file mode 100644
index 0000000000..20a1409a6c
--- /dev/null
+++ b/elf/tst-relr.c
@@ -0,0 +1,20 @@
+static int o, x;
+void *arr[] = {
+ &o, &o, &o, &o, &o, &o, &o, &o, &o, &o, &o, &o, &o, &o, &o, &o,
+ 0,
+ &x, &x, &x, &x, &x, &x, &x, &x, &x, &x, &x, &x, &x, &x, &x, &x,
+};
+
+static int
+do_test (void)
+{
+ for (int i = 0; i < 16; i++)
+ if (arr[i] != &o)
+ return 1;
+ for (int i = 17; i < 33; i++)
+ if (arr[i] != &x)
+ return 1;
+ return 0;
+}
+
+#include <support/test-driver.c>
--
2.33.0.882.g93a45727a2-goog
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