Re: [PATCH v2] elf: Stop with zero entry point value [BZ #28453]

[Siddhesh Poyarekar <siddhesh@gotplt.org>]

On 12/10/21 08:10, H.J. Lu via Libc-alpha wrote:
> Changes in the v2 patch:
> 
> 1. Use rtld_progname in the error message.
> 
> Stop with zero entry point value unless we are tracing shared objects
> since a zero entry point value in the ELF header indicates there is no
> associated entry point.  Now we get
> 
> $ ./elf/ld.so /lib64/libstdc++.so.6.0.29
> ./elf/ld.so: cannot execute '/lib64/libstdc++.so.6.0.29' without entry point
> $
> 
> instead of
> 
> $ /lib64/ld-linux-x86-64.so.2 /lib64/libstdc++.so.6.0.29
> Segmentation fault (core dumped)
> $
> 
> This fixes [BZ #28453].

The change that introduced the zero entry point in binutils:

https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=5226a6a892f922ea672e5775c61776830aaf27b7

and original context of the discussion:

https://bugzilla.redhat.com/show_bug.cgi?id=2004952

LGTM.

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

> ---
>   elf/Makefile            | 10 ++++++++++
>   elf/rtld.c              |  8 ++++++++
>   elf/tst-rtld-run-dso.sh | 33 +++++++++++++++++++++++++++++++++
>   3 files changed, 51 insertions(+)
>   create mode 100755 elf/tst-rtld-run-dso.sh
> 
> diff --git a/elf/Makefile b/elf/Makefile
> index ef36008673..1832dfa537 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -50,6 +50,10 @@ ifeq (yesyes,$(build-shared)$(run-built-tests))
>   tests-special += $(objpfx)list-tunables.out
>   endif
>   
> +ifeq (yes,$(build-shared))
> +tests-special += $(objpfx)tst-rtld-run-dso.out
> +endif
> +
>   # Make sure that the compiler does not insert any library calls in tunables
>   # code paths.
>   ifeq (yes,$(have-loop-to-function))
> @@ -1877,6 +1881,12 @@ $(objpfx)list-tunables.out: tst-rtld-list-tunables.sh $(objpfx)ld.so
>   	    $(objpfx)/tst-rtld-list-tunables.out > $@; \
>   	$(evaluate-test)
>   
> +$(objpfx)tst-rtld-run-dso.out: tst-rtld-run-dso.sh $(objpfx)ld.so \
> +			    $(objpfx)testobj1.so
> +	$(SHELL) tst-rtld-run-dso.sh $(objpfx)ld.so $(objpfx)testobj1.so \
> +	    '$(test-wrapper-env)' '$(run_program_env)' > $@
> +	$(evaluate-test)
> +
>   tst-dst-static-ENV = LD_LIBRARY_PATH='$$ORIGIN'

Test.  OK.

>   
>   $(objpfx)tst-rtld-help.out: $(objpfx)ld.so
> diff --git a/elf/rtld.c b/elf/rtld.c
> index 6ce1e07dc0..b4373ba193 100644
> --- a/elf/rtld.c
> +++ b/elf/rtld.c
> @@ -1424,6 +1424,14 @@ dl_main (const ElfW(Phdr) *phdr,
>   	 implementations which has no real free() function it does not
>   	 makes sense to free the old string first.  */
>         main_map->l_name = (char *) "";
> +
> +      /* Stop if there is no associated entry point and we are not
> +         tracing shared objects.  */
> +      if (main_map->l_entry == main_map->l_addr
> +	  && state.mode != rtld_mode_trace)
> +	_dl_fatal_printf("%s: cannot execute '%s' without entry point\n",
> +			 ld_so_name, rtld_progname);
> +

Bail out if the entry point is same as load address.  OK.

>         *user_entry = main_map->l_entry;
>   
>         /* Set bit indicating this is the main program map.  */
> diff --git a/elf/tst-rtld-run-dso.sh b/elf/tst-rtld-run-dso.sh
> new file mode 100755
> index 0000000000..5192f64210
> --- /dev/null
> +++ b/elf/tst-rtld-run-dso.sh
> @@ -0,0 +1,33 @@
> +#!/bin/sh
> +# Test for ld.so on a shared library with no associated entry point.
> +# 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/>.
> +
> +set -e
> +
> +rtld=$1
> +dso=$2
> +test_wrapper_env=$3
> +run_program_env=$4
> +
> +LC_ALL=C
> +export LC_ALL
> +
> +${test_wrapper_env} \
> +${run_program_env} \
> +$rtld $dso 2>&1 \
> +| grep "cannot execute"
> 

OK.

Thanks,
Siddhesh