[v3] Support multiple .eh_frame sections
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Commit Message
This patch is based on MULTIPLE_FRAME_SECTIONS and EH_FRAME_LINKONCE,
it allows backend to enable this feature and use '--gc-sections' simply.
* gas/dw2gencfi.h (TARGET_MULTIPLE_EH_FRAME_SECTIONS): New.
(MULTIPLE_FRAME_SECTIONS): Add TARGET_MULTIPLE_EH_FRAME_SECTIONS.
* gas/dw2gencfi.c (EH_FRAME_LINKONCE): Add TARGET_MULTIPLE_EH_FRAME_SECTIONS.
(is_now_linkonce_segment): Likewise.
(get_cfi_seg): Create relocation info between .eh_frame.* and .text.* section.
* bfd/elf-bfd.h (elf_backend_can_make_multiple_eh_frame): New.
* bfd/elfxx-target.h (elf_backend_can_make_multiple_eh_frame): Likewise.
* bfd/elflink.c (_bfd_elf_default_action_discarded): Add checking for
elf_backend_can_make_multiple_eh_frame.
---
bfd/elf-bfd.h | 3 +++
bfd/elflink.c | 7 +++++++
bfd/elfxx-target.h | 4 ++++
gas/doc/internals.texi | 4 ++++
gas/dw2gencfi.c | 27 +++++++++++++++++++++++++--
gas/dw2gencfi.h | 7 ++++++-
6 files changed, 49 insertions(+), 3 deletions(-)
Comments
Hi Jojo,
> This patch is based on MULTIPLE_FRAME_SECTIONS and EH_FRAME_LINKONCE,
> it allows backend to enable this feature and use '--gc-sections' simply.
>
> * gas/dw2gencfi.h (TARGET_MULTIPLE_EH_FRAME_SECTIONS): New.
> (MULTIPLE_FRAME_SECTIONS): Add TARGET_MULTIPLE_EH_FRAME_SECTIONS.
> * gas/dw2gencfi.c (EH_FRAME_LINKONCE): Add TARGET_MULTIPLE_EH_FRAME_SECTIONS.
> (is_now_linkonce_segment): Likewise.
> (get_cfi_seg): Create relocation info between .eh_frame.* and .text.* section.
>
> * bfd/elf-bfd.h (elf_backend_can_make_multiple_eh_frame): New.
> * bfd/elfxx-target.h (elf_backend_can_make_multiple_eh_frame): Likewise.
> * bfd/elflink.c (_bfd_elf_default_action_discarded): Add checking for
> elf_backend_can_make_multiple_eh_frame.
Approved - please apply.
Cheers
Nick
On Thu, Nov 3, 2022 at 3:26 AM Nick Clifton via Binutils
<binutils@sourceware.org> wrote:
>
> Hi Jojo,
>
> > This patch is based on MULTIPLE_FRAME_SECTIONS and EH_FRAME_LINKONCE,
> > it allows backend to enable this feature and use '--gc-sections' simply.
> >
> > * gas/dw2gencfi.h (TARGET_MULTIPLE_EH_FRAME_SECTIONS): New.
> > (MULTIPLE_FRAME_SECTIONS): Add TARGET_MULTIPLE_EH_FRAME_SECTIONS.
> > * gas/dw2gencfi.c (EH_FRAME_LINKONCE): Add TARGET_MULTIPLE_EH_FRAME_SECTIONS.
> > (is_now_linkonce_segment): Likewise.
> > (get_cfi_seg): Create relocation info between .eh_frame.* and .text.* section.
> >
> > * bfd/elf-bfd.h (elf_backend_can_make_multiple_eh_frame): New.
> > * bfd/elfxx-target.h (elf_backend_can_make_multiple_eh_frame): Likewise.
> > * bfd/elflink.c (_bfd_elf_default_action_discarded): Add checking for
> > elf_backend_can_make_multiple_eh_frame.
>
>
> Approved - please apply.
>
> Cheers
> Nick
>
Hi Jojo, is there a test demonstrating the handling of multiple
.eh_frame? Input .eh_frame or output .eh_frame?
.eh_frame handling is known to be weird with --gc-sections but linkers
have gained support:
https://maskray.me/blog/2020-11-08-stack-unwinding#linker-perspective
Since monolithic .eh_frame has been around for so long, some tools may
gain reliance on it and they will need to be fixed.
Also, it'd be nice to verify that .eh_frame_hdr and PT_GNU_EH_FRAME
creation is still correct.
在 2022/11/4 03:34, Fangrui Song 写道:
> On Thu, Nov 3, 2022 at 3:26 AM Nick Clifton via Binutils
> <binutils@sourceware.org> wrote:
>> Hi Jojo,
>>
>>> This patch is based on MULTIPLE_FRAME_SECTIONS and EH_FRAME_LINKONCE,
>>> it allows backend to enable this feature and use '--gc-sections' simply.
>>>
>>> * gas/dw2gencfi.h (TARGET_MULTIPLE_EH_FRAME_SECTIONS): New.
>>> (MULTIPLE_FRAME_SECTIONS): Add TARGET_MULTIPLE_EH_FRAME_SECTIONS.
>>> * gas/dw2gencfi.c (EH_FRAME_LINKONCE): Add TARGET_MULTIPLE_EH_FRAME_SECTIONS.
>>> (is_now_linkonce_segment): Likewise.
>>> (get_cfi_seg): Create relocation info between .eh_frame.* and .text.* section.
>>>
>>> * bfd/elf-bfd.h (elf_backend_can_make_multiple_eh_frame): New.
>>> * bfd/elfxx-target.h (elf_backend_can_make_multiple_eh_frame): Likewise.
>>> * bfd/elflink.c (_bfd_elf_default_action_discarded): Add checking for
>>> elf_backend_can_make_multiple_eh_frame.
>>
>> Approved - please apply.
>>
>> Cheers
>> Nick
>>
> Hi Jojo, is there a test demonstrating the handling of multiple
> .eh_frame? Input .eh_frame or output .eh_frame?
>
> .eh_frame handling is known to be weird with --gc-sections but linkers
> have gained support:
> https://maskray.me/blog/2020-11-08-stack-unwinding#linker-perspective
We want to encode another eh_frame format like arm, and based on
seprated eh_frame sections :)
> Since monolithic .eh_frame has been around for so long, some tools may
> gain reliance on it and they will need to be fixed.
> Also, it'd be nice to verify that .eh_frame_hdr and PT_GNU_EH_FRAME
> creation is still correct.
Thanks for your mention, it looks good for baremetal.
.eh_frame_hdr or shared libs system like linux is not supported by now,
we will append another patch later if we finished that.
@@ -1432,6 +1432,9 @@ struct elf_backend_data
bool (*elf_backend_can_make_lsda_relative_eh_frame)
(bfd *, struct bfd_link_info *, asection *);
+ /* Tell linker to support multiple eh_frame sections. */
+ bool elf_backend_can_make_multiple_eh_frame;
+
/* This function returns an encoding after computing the encoded
value (and storing it in ENCODED) for the given OFFSET into OSEC,
to be stored in at LOC_OFFSET into the LOC_SEC input section.
@@ -10924,12 +10924,19 @@ elf_section_ignore_discarded_relocs (asection *sec)
unsigned int
_bfd_elf_default_action_discarded (asection *sec)
{
+ const struct elf_backend_data *bed;
+ bed = get_elf_backend_data (sec->owner);
+
if (sec->flags & SEC_DEBUGGING)
return PRETEND;
if (strcmp (".eh_frame", sec->name) == 0)
return 0;
+ if (bed->elf_backend_can_make_multiple_eh_frame
+ && strncmp (sec->name, ".eh_frame.", 10) == 0)
+ return 0;
+
if (strcmp (".gcc_except_table", sec->name) == 0)
return 0;
@@ -658,6 +658,9 @@
#ifndef elf_backend_can_make_lsda_relative_eh_frame
#define elf_backend_can_make_lsda_relative_eh_frame _bfd_elf_can_make_relative
#endif
+#ifndef elf_backend_can_make_multiple_eh_frame
+#define elf_backend_can_make_multiple_eh_frame 0
+#endif
#ifndef elf_backend_encode_eh_address
#define elf_backend_encode_eh_address _bfd_elf_encode_eh_address
#endif
@@ -891,6 +894,7 @@ static const struct elf_backend_data elfNN_bed =
elf_backend_eh_frame_address_size,
elf_backend_can_make_relative_eh_frame,
elf_backend_can_make_lsda_relative_eh_frame,
+ elf_backend_can_make_multiple_eh_frame,
elf_backend_encode_eh_address,
elf_backend_write_section,
elf_backend_elfsym_local_is_section,
@@ -1581,6 +1581,10 @@ If defined, GAS will check this macro before performing any optimizations on
the DWARF call frame debug information that is emitted. Targets which
implement link time relaxation may need to define this macro and set it to zero
if it is possible to change the size of a function's prologue.
+
+@item TARGET_MULTIPLE_EH_FRAME_SECTIONS
+If defined, GAS will create multiple .eh_frame.* sections according to
+the name of owner's function sections.
@end table
@node Object format backend
@@ -75,7 +75,8 @@
# define tc_cfi_endproc(fde) ((void) (fde))
#endif
-#define EH_FRAME_LINKONCE (SUPPORT_FRAME_LINKONCE || compact_eh)
+#define EH_FRAME_LINKONCE (SUPPORT_FRAME_LINKONCE || compact_eh \
+ || TARGET_MULTIPLE_EH_FRAME_SECTIONS)
#ifndef DWARF2_FORMAT
#define DWARF2_FORMAT(SEC) dwarf2_format_32bit
@@ -277,6 +278,9 @@ is_now_linkonce_segment (void)
if (compact_eh)
return now_seg;
+ if (TARGET_MULTIPLE_EH_FRAME_SECTIONS)
+ return now_seg;
+
if ((bfd_section_flags (now_seg)
& (SEC_LINK_ONCE | SEC_LINK_DUPLICATES_DISCARD
| SEC_LINK_DUPLICATES_ONE_ONLY | SEC_LINK_DUPLICATES_SAME_SIZE
@@ -1333,14 +1337,33 @@ static segT
get_cfi_seg (segT cseg, const char *base, flagword flags, int align)
{
/* Exclude .debug_frame sections for Compact EH. */
- if (SUPPORT_FRAME_LINKONCE || ((flags & SEC_DEBUGGING) == 0 && compact_eh))
+ if (SUPPORT_FRAME_LINKONCE || ((flags & SEC_DEBUGGING) == 0 && compact_eh)
+ || ((flags & SEC_DEBUGGING) == 0 && TARGET_MULTIPLE_EH_FRAME_SECTIONS))
{
+ segT iseg = cseg;
struct dwcfi_seg_list *l;
l = dwcfi_hash_find_or_make (cseg, base, flags);
cseg = l->seg;
subseg_set (cseg, l->subseg);
+
+ if (TARGET_MULTIPLE_EH_FRAME_SECTIONS
+ && (flags & DWARF2_EH_FRAME_READ_ONLY))
+ {
+ const frchainS *ifrch = seg_info (iseg)->frchainP;
+ const frchainS *frch = seg_info (cseg)->frchainP;
+ expressionS exp;
+
+ exp.X_op = O_symbol;
+ exp.X_add_symbol = (symbolS *) local_symbol_make (cseg->name, cseg, frch->frch_root, 0);
+ exp.X_add_number = 0;
+ subseg_set (iseg, ifrch->frch_subseg);
+ fix_new_exp (ifrch->frch_root, 0, 0, &exp, 0, BFD_RELOC_NONE);
+
+ /* Restore the original segment info. */
+ subseg_set (cseg, l->subseg);
+ }
}
else
{
@@ -66,7 +66,12 @@ extern void cfi_add_CFA_restore_state (void);
#define SUPPORT_COMPACT_EH 0
#endif
-#define MULTIPLE_FRAME_SECTIONS (SUPPORT_FRAME_LINKONCE || SUPPORT_COMPACT_EH)
+#ifndef TARGET_MULTIPLE_EH_FRAME_SECTIONS
+#define TARGET_MULTIPLE_EH_FRAME_SECTIONS 0
+#endif
+
+#define MULTIPLE_FRAME_SECTIONS (SUPPORT_FRAME_LINKONCE || SUPPORT_COMPACT_EH \
+ || TARGET_MULTIPLE_EH_FRAME_SECTIONS)
struct cfi_insn_data
{