[PATCH v6 1/5] elf: Support DT_RELR relative relocation format [BZ #27924]
H.J. Lu
hjl.tools@gmail.com
Tue Mar 29 22:34:26 GMT 2022
On Tue, Mar 29, 2022 at 9:34 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 10/03/2022 17:03, H.J. Lu via Libc-alpha 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.
>
> I really don't think ia64 ELFCLASS32 should be blocker for this change.
>
> >
> > * 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:
>
> Agreed.
>
> >
> > * 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.
> > 6. Move ELF_DYNAMIC_DO_RELR before ELF_DYNAMIC_DO_REL.
>
> Patch looks ok, some comments below.
>
> > ---
> > 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
> > +
>
> I think you don't need to setup have_dt_relr, it does not accomplish anything here
> (LIBC_CONFIG_VAR already sets the required have-dt-relr on config.make).
Fixed in v7.
> > 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
>
> Please use one line per test:
>
> tests += \
> tst-relr \
> tst-relr-pie \
> # tests
> tests-pie +=
> tst-relr-pie \
> # tests-pie
> tests-special += \
> $(objpfx)check-tst-relr-pie.out \
> # tests-special
>
> Also, the tst-rel-pie should only be added if $(have-fpie):
>
> ifeq ($(have-fpie),yes)
> tests-pie += \
> tst-relr-pie \
> #tests-pie
> endf
Fixed in v7.
> > +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)
>
> Ok.
>
> > diff --git a/elf/dynamic-link.h b/elf/dynamic-link.h
> > index 25dd7ca4f2..d04c457e55 100644
> > --- a/elf/dynamic-link.h
> > +++ b/elf/dynamic-link.h
> > @@ -146,12 +146,46 @@ 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)); \
> > + if (((map) != &GL(dl_rtld_map) || DO_RTLD_BOOTSTRAP)) \
> > + ELF_DYNAMIC_DO_RELR (map); \
> > ELF_DYNAMIC_DO_REL ((map), (scope), edr_lazy, skip_ifunc); \
> > ELF_DYNAMIC_DO_RELA ((map), (scope), edr_lazy, skip_ifunc); \
> > } while (0)
>
> Ok.
>
> > 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;
> >
>
> Ok.
>
> > +/* 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 */
>
> Ok.
>
> > 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);
>
> Ok.
>
> > 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
>
> Ok.
>
> > 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++)
>
> Use array_length here.
Fixed in v7.
> > + 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>
>
> This fails without the rest of the patchset applied:
>
> $ ./testrun.sh elf/tst-relr
> elf/tst-relr: ./libc.so.6: version `GLIBC_ABI_DT_RELR' not found (required by elf/tst-relr)
>
> I suggest you move it to third patch when GLIBC_ABI_DT_RELR is added.
Fixed in v7.
The first patch just defines DT_RELR related macros and types. The
second patch adds GLIBC_ABI_DT_RELR.
Thanks.
--
H.J.
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