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Re: [PATCH] Always keep sections marked with SEC_KEEP
- From: Alan Modra <amodra at gmail dot com>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: Binutils <binutils at sourceware dot org>
- Date: Fri, 23 Oct 2015 21:38:06 +1030
- Subject: Re: [PATCH] Always keep sections marked with SEC_KEEP
- Authentication-results: sourceware.org; auth=none
- References: <1445541893-8228-1-git-send-email-hjl dot tools at gmail dot com> <20151022235157 dot GS13961 at bubble dot grove dot modra dot org> <CAMe9rOqcmqQ6B5_ufLgKKTHnm6s7oFqT2PnctHTr=U6OBqOeZg at mail dot gmail dot com>
On Fri, Oct 23, 2015 at 03:47:38AM -0700, H.J. Lu wrote:
> On Thu, Oct 22, 2015 at 4:51 PM, Alan Modra <amodra@gmail.com> wrote:
> > On Thu, Oct 22, 2015 at 12:24:53PM -0700, H.J. Lu wrote:
> >> diff --git a/bfd/elflink.c b/bfd/elflink.c
> >> index 73fe469..06df821 100644
> >> --- a/bfd/elflink.c
> >> +++ b/bfd/elflink.c
> >> @@ -12449,7 +12449,8 @@ elf_gc_sweep (bfd *abfd, struct bfd_link_info *info)
> >> o->gc_mark = first->gc_mark;
> >> }
> >>
> >> - if (o->gc_mark)
> >> + /* Always keep sections marked with SEC_KEEP. */
> >> + if (o->gc_mark || (o->flags & SEC_KEEP))
> >> continue;
> >>
> >> /* Skip sweeping sections already excluded. */
> >
> > This is wrong. You're breaking the gc-sections model (and ignoring
> > the special combination of both SEC_EXCLUDE and SEC_KEEP). Sections
> > with SEC_KEEP are supposed to have gc_mark set by the call to
> > _bfd_elf_gc_mark in bfd_elf_gc_sections.
>
> Do you have a testcase to show the regression my patch caused?
No, but my point about breaking the gc-sections model is that if you
need to test SEC_KEEP in elf_gc_sweep then you have a section that has
not had its relocations examined for further sections that might need
to be kept.
> If both SEC_EXCLUDE and SEC_KEEP are set, my patch doesn't
> change the behavior of elf_gc_sweep.
Sorry, yes, you might be right about the combination of those flags.
BTW, if we want to keep all input sections matching a __start_* or
__stop_* symbol reference then something like the following should
work. What do you think?
diff --git a/bfd/elf-bfd.h b/bfd/elf-bfd.h
index eba32b5..df2d86a 100644
--- a/bfd/elf-bfd.h
+++ b/bfd/elf-bfd.h
@@ -2302,7 +2302,7 @@ extern asection *_bfd_elf_gc_mark_hook
extern asection *_bfd_elf_gc_mark_rsec
(struct bfd_link_info *, asection *, elf_gc_mark_hook_fn,
- struct elf_reloc_cookie *);
+ struct elf_reloc_cookie *, bfd_boolean *);
extern bfd_boolean _bfd_elf_gc_mark_reloc
(struct bfd_link_info *, asection *, elf_gc_mark_hook_fn,
diff --git a/bfd/elf-eh-frame.c b/bfd/elf-eh-frame.c
index 7d65dae..e303189 100644
--- a/bfd/elf-eh-frame.c
+++ b/bfd/elf-eh-frame.c
@@ -902,7 +902,8 @@ _bfd_elf_parse_eh_frame (bfd *abfd, struct bfd_link_info *info,
REQUIRE (GET_RELOC (buf));
/* Chain together the FDEs for each section. */
- rsec = _bfd_elf_gc_mark_rsec (info, sec, gc_mark_hook, cookie);
+ rsec = _bfd_elf_gc_mark_rsec (info, sec, gc_mark_hook,
+ cookie, NULL);
/* RSEC will be NULL if FDE was cleared out as it was belonging to
a discarded SHT_GROUP. */
if (rsec)
diff --git a/bfd/elflink.c b/bfd/elflink.c
index 0bec93d..c3b537b 100644
--- a/bfd/elflink.c
+++ b/bfd/elflink.c
@@ -12074,8 +12074,6 @@ _bfd_elf_gc_mark_hook (asection *sec,
struct elf_link_hash_entry *h,
Elf_Internal_Sym *sym)
{
- const char *sec_name;
-
if (h != NULL)
{
switch (h->root.type)
@@ -12087,33 +12085,6 @@ _bfd_elf_gc_mark_hook (asection *sec,
case bfd_link_hash_common:
return h->root.u.c.p->section;
- case bfd_link_hash_undefined:
- case bfd_link_hash_undefweak:
- /* To work around a glibc bug, keep all XXX input sections
- when there is an as yet undefined reference to __start_XXX
- or __stop_XXX symbols. The linker will later define such
- symbols for orphan input sections that have a name
- representable as a C identifier. */
- if (strncmp (h->root.root.string, "__start_", 8) == 0)
- sec_name = h->root.root.string + 8;
- else if (strncmp (h->root.root.string, "__stop_", 7) == 0)
- sec_name = h->root.root.string + 7;
- else
- sec_name = NULL;
-
- if (sec_name && *sec_name != '\0')
- {
- bfd *i;
-
- for (i = info->input_bfds; i; i = i->link.next)
- {
- sec = bfd_get_section_by_name (i, sec_name);
- if (sec)
- return sec;
- }
- }
- break;
-
default:
break;
}
@@ -12131,7 +12102,8 @@ _bfd_elf_gc_mark_hook (asection *sec,
asection *
_bfd_elf_gc_mark_rsec (struct bfd_link_info *info, asection *sec,
elf_gc_mark_hook_fn gc_mark_hook,
- struct elf_reloc_cookie *cookie)
+ struct elf_reloc_cookie *cookie,
+ bfd_boolean *start_stop)
{
unsigned long r_symndx;
struct elf_link_hash_entry *h;
@@ -12160,6 +12132,38 @@ _bfd_elf_gc_mark_rsec (struct bfd_link_info *info, asection *sec,
handling copy relocs. */
if (h->u.weakdef != NULL)
h->u.weakdef->mark = 1;
+
+ if (start_stop != NULL
+ && (h->root.type == bfd_link_hash_undefined
+ || h->root.type == bfd_link_hash_undefweak))
+ {
+ /* To work around a glibc bug, mark all XXX input sections
+ when there is an as yet undefined reference to __start_XXX
+ or __stop_XXX symbols. The linker will later define such
+ symbols for orphan input sections that have a name
+ representable as a C identifier. */
+ const char *sec_name = NULL;
+ if (strncmp (h->root.root.string, "__start_", 8) == 0)
+ sec_name = h->root.root.string + 8;
+ else if (strncmp (h->root.root.string, "__stop_", 7) == 0)
+ sec_name = h->root.root.string + 7;
+
+ if (sec_name != NULL && *sec_name != '\0')
+ {
+ bfd *i;
+
+ for (i = info->input_bfds; i != NULL; i = i->link.next)
+ {
+ asection *s = bfd_get_section_by_name (i, sec_name);
+ if (s != NULL && !s->gc_mark)
+ {
+ *start_stop = TRUE;
+ return s;
+ }
+ }
+ }
+ }
+
return (*gc_mark_hook) (sec, info, cookie->rel, h, NULL);
}
@@ -12178,15 +12182,39 @@ _bfd_elf_gc_mark_reloc (struct bfd_link_info *info,
struct elf_reloc_cookie *cookie)
{
asection *rsec;
+ bfd_boolean start_stop = FALSE;
- rsec = _bfd_elf_gc_mark_rsec (info, sec, gc_mark_hook, cookie);
- if (rsec && !rsec->gc_mark)
+ rsec = _bfd_elf_gc_mark_rsec (info, sec, gc_mark_hook, cookie, &start_stop);
+ while (rsec != NULL)
{
- if (bfd_get_flavour (rsec->owner) != bfd_target_elf_flavour
- || (rsec->owner->flags & DYNAMIC) != 0)
- rsec->gc_mark = 1;
- else if (!_bfd_elf_gc_mark (info, rsec, gc_mark_hook))
- return FALSE;
+ asection *s;
+
+ if (!rsec->gc_mark)
+ {
+ if (bfd_get_flavour (rsec->owner) != bfd_target_elf_flavour
+ || (rsec->owner->flags & DYNAMIC) != 0)
+ rsec->gc_mark = 1;
+ else if (!_bfd_elf_gc_mark (info, rsec, gc_mark_hook))
+ return FALSE;
+ }
+ if (!start_stop)
+ break;
+ s = bfd_get_next_section_by_name (rsec);
+ if (s == NULL)
+ {
+ bfd *i = rsec->owner;
+
+ if (i != NULL)
+ {
+ while ((i = i->link.next) != NULL)
+ {
+ s = bfd_get_section_by_name (i, rsec->name);
+ if (s != NULL)
+ break;
+ }
+ }
+ }
+ rsec = s;
}
return TRUE;
}
--
Alan Modra
Australia Development Lab, IBM