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Re: [PATCH 5/7] elf: Enable relro for static build
- From: Florian Weimer <fw at deneb dot enyo dot de>
- To: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- Cc: libc-alpha at sourceware dot org
- Date: Sun, 01 Dec 2019 10:55:49 +0100
- Subject: Re: [PATCH 5/7] elf: Enable relro for static build
- References: <20191129210327.26434-1-adhemerval.zanella@linaro.org> <20191129210327.26434-5-adhemerval.zanella@linaro.org>
* Adhemerval Zanella:
> The code is similar to the one at rtld.c, where its check for the
> PT_GNU_RELRO header values from program headers and call
> _dl_protected_relro with the updated l_relro_{addr,size} values.
This is not the actual code that does RELRO in most cases, it's only
used with prelink. _dl_relocate_object is what is used.
> diff --git a/elf/dl-support.c b/elf/dl-support.c
> index 5526d5ee6e..bdb5c2ae91 100644
> --- a/elf/dl-support.c
> +++ b/elf/dl-support.c
> @@ -367,14 +367,24 @@ _dl_non_dynamic_init (void)
> if (_dl_platform != NULL)
> _dl_platformlen = strlen (_dl_platform);
>
> - /* Scan for a program header telling us the stack is nonexecutable. */
> if (_dl_phdr != NULL)
> - for (uint_fast16_t i = 0; i < _dl_phnum; ++i)
> - if (_dl_phdr[i].p_type == PT_GNU_STACK)
> + for (const ElfW(Phdr) *ph = _dl_phdr; ph < &_dl_phdr[_dl_phnum]; ++ph)
> + switch (ph->p_type)
> {
> - _dl_stack_flags = _dl_phdr[i].p_flags;
> + /* Check if the stack is nonexecutable. */
> + case PT_GNU_STACK:
> + _dl_stack_flags = ph->p_flags;
> + break;
> +
> + case PT_GNU_RELRO:
> + _dl_main_map.l_relro_addr = ph->p_vaddr;
> + _dl_main_map.l_relro_size = ph->p_memsz;
> break;
> }
> +
> + /* Setup relro on the binary itself. */
> + if (_dl_main_map.l_relro_size)
> + _dl_protect_relro (&_dl_main_map);
Please use an explicit comparison with != 0.
I have a test case for this which I can post. Somewhat bizarrely,
full RELRO for statically linked binaries requires linking with -z now.