As noted in bug 28475, the access attribute on memfrob in <string.h>
is incorrect: the function both reads and writes the memory pointed to
by its argument, so it needs to use __read_write__, not
__write_only__. This incorrect attribute results in a build failure
for accessing uninitialized memory for s390x-linux-gnu-O3 with
build-many-glibcs.py using GCC mainline.
Correct the attribute. Fixing this shows up that some calls to
memfrob in elf/ tests are reading uninitialized memory; I'm not
entirely sure of the purpose of those calls, but guess they are about
ensuring that the stack space is indeed allocated at that point in the
function, and so it matters that they are calling a function whose
semantics are unknown to the compiler. Thus, change the first memfrob
call in those tests to use explicit_bzero instead, as suggested by
Florian in
<https://sourceware.org/pipermail/libc-alpha/2021-October/132119.html>,
to avoid the use of uninitialized memory.
Tested for x86_64, and with build-many-glibcs.py (GCC mainline) for
s390x-linux-gnu-O3.
calling _dl_start, using GOTOFF access.
* configure.in (libc_cv_z_execstack): New check.
* configure: Regenerated.
* config.make.in (have-z-execstack): New variable set by it.
* elf/tst-execstack.c: New file.
* elf/tst-execstack-mod.c: New file.
* elf/tst-execstack-needed.c: New file.
* elf/Makefile (tests-execstack-yes, modules-execstack-yes): New
variables.
(tests, modules-names): Conditionally include them.
(distribute): Add tst-execstack-mod.c here.
($(objpfx)tst-execstack, $(objpfx)tst-execstack.out): New targets.
(LDFLAGS-tst-execstack, LDFLAGS-tst-execstack-mod): New variables.
($(objpfx)tst-execstack-needed): New target.
(LDFLAGS-tst-execstack-needed): New variable.
* sysdeps/generic/ldsodefs.h (struct rtld_global): Add _dl_stack_flags
and _dl_make_stack_executable_hook. Declare _dl_make_stack_executable.
* elf/rtld.c (_rtld_global): Add initializer for _dl_stack_flags.
(dl_main): Reset _dl_stack_flags according to PT_GNU_STACK phdr.
Initialize _dl_make_stack_executable_hook.
* elf/dl-support.c: Define those new variables.
(_dl_non_dynamic_init): Scan phdrs for PT_GNU_STACK.
(_dl_phdr): Fix type.
* elf/dl-load.c (_dl_map_object_from_fd): Grok PT_GNU_STACK phdr and
enable execute permission for the stack if necessary.
* sysdeps/generic/dl-execstack.c: New file.
* elf/Makefile (dl-routines): Add it.
* elf/Versions (ld: GLIBC_PRIVATE): Add _dl_make_stack_executable.
* sysdeps/unix/sysv/linux/dl-execstack.c: New file.
