[yocto] [prelink-cross][PATCH] Support copy relocations in .data.rel.ro
Kyle Russell
bkylerussell at gmail.com
Thu Oct 4 07:12:17 PDT 2018
Hey Mark,
Do you think this approach is reasonable? If so, I have another patch I'd
like to propose that would enable us to better catch error scenarios (like
the last two patches address) that we might encounter during
do_image_prelink. We just happened to detect these last two issues even
though image_prelink didn't fail the build, so I'd like to provide an
option to enable a little more assertive error path, if desired.
Thanks,
Kyle
On Fri, Sep 28, 2018 at 10:57 AM Kyle Russell <bkylerussell at gmail.com>
wrote:
> binutils-2.28 (17026142ef35b62ac88bfe517b4160614902cb28) adds support
> for copying read-only dynamic symbols into .data.rel.ro instead of .bss
> since .bss is technically writable. This causes prelink to error out on
> any binary containing COPY relocations in .data.rel.ro.
>
> Read-only variables defined in shared libraries should be copied directly
> into the space allocated for them in .data.rel.ro by the linker.
>
> To achieve this, we determine whether either of the two sections
> containing copy relocations is .data.rel.ro. If so, we relocate the
> symbol memory directly into the existing section instead of constructing
> a new .(s)dynbss section once prelink_build_conflicts() returns.
>
> Fixes cxx1.sh, cxx2.sh, and cxx3.sh on Fedora 28 (which uses
> binutils-2.29).
> ---
> src/conflict.c | 51 +++++++++++++++++++++++++++++++++++++-------------
> src/undo.c | 9 +++++++++
> 2 files changed, 47 insertions(+), 13 deletions(-)
>
> diff --git a/src/conflict.c b/src/conflict.c
> index 9ae2ddb..5613ace 100644
> --- a/src/conflict.c
> +++ b/src/conflict.c
> @@ -450,7 +450,7 @@ get_relocated_mem (struct prelink_info *info, DSO
> *dso, GElf_Addr addr,
> int
> prelink_build_conflicts (struct prelink_info *info)
> {
> - int i, ndeps = info->ent->ndepends + 1;
> + int i, reset_dynbss = 0, reset_sdynbss = 0, ndeps = info->ent->ndepends
> + 1;
> struct prelink_entry *ent;
> int ret = 0;
> DSO *dso;
> @@ -675,6 +675,11 @@ prelink_build_conflicts (struct prelink_info *info)
> dso->filename);
> goto error_out;
> }
> +
> + name = strptr (dso, dso->ehdr.e_shstrndx,
> dso->shdr[bss1].sh_name);
> + if (strcmp(name, ".data.rel.ro") == 0)
> + reset_sdynbss = 1;
> +
> firstbss2 = i;
> info->sdynbss_size = cr.rela[i - 1].r_offset -
> cr.rela[0].r_offset;
> info->sdynbss_size += cr.rela[i - 1].r_addend;
> @@ -702,6 +707,10 @@ prelink_build_conflicts (struct prelink_info *info)
> }
> }
>
> + name = strptr (dso, dso->ehdr.e_shstrndx, dso->shdr[bss2].sh_name);
> + if (strcmp(name, ".data.rel.ro") == 0)
> + reset_dynbss = 1;
> +
> info->dynbss_size = cr.rela[cr.count - 1].r_offset
> - cr.rela[firstbss2].r_offset;
> info->dynbss_size += cr.rela[cr.count - 1].r_addend;
> @@ -719,9 +728,9 @@ prelink_build_conflicts (struct prelink_info *info)
> && strcmp (name = strptr (dso, dso->ehdr.e_shstrndx,
> dso->shdr[bss1].sh_name),
> ".dynbss") != 0
> - && strcmp (name, ".sdynbss") != 0)
> + && strcmp (name, ".sdynbss") != 0 && strcmp (name, ".data.rel.ro")
> != 0)
> {
> - error (0, 0, "%s: COPY relocations don't point into .bss or
> .sbss section",
> + error (0, 0, "%s: COPY relocations don't point into .bss, .sbss,
> or .data.rel.ro sections",
> dso->filename);
> goto error_out;
> }
> @@ -730,9 +739,9 @@ prelink_build_conflicts (struct prelink_info *info)
> && strcmp (name = strptr (dso, dso->ehdr.e_shstrndx,
> dso->shdr[bss2].sh_name),
> ".dynbss") != 0
> - && strcmp (name, ".sdynbss") != 0)
> + && strcmp (name, ".sdynbss") != 0 && strcmp (name, ".data.rel.ro")
> != 0)
> {
> - error (0, 0, "%s: COPY relocations don't point into .bss or
> .sbss section",
> + error (0, 0, "%s: COPY relocations don't point into .bss, .sbss,
> or .data.rel.ro section",
> dso->filename);
> goto error_out;
> }
> @@ -768,16 +777,21 @@ prelink_build_conflicts (struct prelink_info *info)
> }
>
> assert (j < ndeps);
> + GElf_Addr symaddr = s->u.ent->base + s->value;
> + char *buf;
> +
> if (i < firstbss2)
> - j = get_relocated_mem (info, ndso, s->u.ent->base + s->value,
> - info->sdynbss + cr.rela[i].r_offset
> - - info->sdynbss_base,
> cr.rela[i].r_addend,
> - cr.rela[i].r_offset);
> + if (reset_sdynbss)
> + buf = get_data(dso, cr.rela[i].r_offset, NULL, NULL);
> + else
> + buf = info->sdynbss + cr.rela[i].r_offset -
> info->sdynbss_base;
> else
> - j = get_relocated_mem (info, ndso, s->u.ent->base + s->value,
> - info->dynbss + cr.rela[i].r_offset
> - - info->dynbss_base,
> cr.rela[i].r_addend,
> - cr.rela[i].r_offset);
> + if (reset_dynbss)
> + buf = get_data(dso, cr.rela[i].r_offset, NULL, NULL);
> + else
> + buf = info->dynbss + cr.rela[i].r_offset - info->dynbss_base;
> +
> + j = get_relocated_mem (info, ndso, symaddr, buf,
> cr.rela[i].r_addend, cr.rela[i].r_offset);
>
> switch (j)
> {
> @@ -815,6 +829,17 @@ prelink_build_conflicts (struct prelink_info *info)
> if (info->dsos[i])
> close_dso (info->dsos[i]);
>
> + if (reset_sdynbss)
> + {
> + free (info->sdynbss);
> + info->sdynbss = NULL;
> + }
> +
> + if (reset_dynbss)
> + {
> + free (info->dynbss);
> + info->dynbss = NULL;
> + }
> info->dsos = NULL;
> free (cr.rela);
> return ret;
> diff --git a/src/undo.c b/src/undo.c
> index 8a55bf2..4c38dab 100644
> --- a/src/undo.c
> +++ b/src/undo.c
> @@ -633,6 +633,15 @@ prelink_undo (DSO *dso)
> d->d_buf = NULL;
> dso->shdr[i].sh_type = SHT_NOBITS;
> }
> + else if (dso->shdr[i].sh_type == SHT_PROGBITS
> + && strcmp (name, ".data.rel.ro") == 0)
> + {
> + scn = dso->scn[i];
> + d = elf_getdata (scn, NULL);
> + assert (d != NULL && elf_getdata (scn, d) == NULL);
> + assert (d->d_size == dso->shdr[i].sh_size);
> + assert (memset(d->d_buf, 0, d->d_size) == d->d_buf);
> + }
> else if (dso->shdr[i].sh_type == SHT_RELA
> && shdr[i].sh_type == SHT_REL)
> {
> --
> 2.17.1
>
>
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