ld: Issue an error for GC on __patchable_function_entries section

Message ID 20200207022328.GA444595@gmail.com
State New
Headers show
Series
  • ld: Issue an error for GC on __patchable_function_entries section
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Commit Message

H.J. Lu Feb. 7, 2020, 2:23 a.m.
On Sun, Feb 02, 2020 at 05:17:48PM -0800, H.J. Lu wrote:
> On Sun, Feb 2, 2020 at 4:57 PM Fangrui Song <i@maskray.me> wrote:

> >

> > On 2020-02-02, H.J. Lu wrote:

> > >On Sat, Feb 1, 2020 at 10:21 AM Fangrui Song <i@maskray.me> wrote:

> > >>

> > >> On 2020-02-01, H.J. Lu wrote:

> > >> >On Sat, Feb 1, 2020 at 9:34 AM Fangrui Song <i@maskray.me> wrote:

> > >> >>

> > >> >> On 2020-02-01, H.J. Lu wrote:

> > >> >> >After all text sections have been garbage collected, if a

> > >> >> >__patchable_function_entries section references a section which

> > >> >> >wasn't marked, mark it with SEC_EXCLUDE and return NULL.  Otherwise,

> > >> >> >keep it.

> > >> >> >

> > >> >> >Should it be handled in _bfd_elf_gc_mark_extra_sections?

> > >> >>

> > >> >> Thanks for paying attention to these feature requests.

> > >> >>

> > >> >> I referenced GNU as and ld requests at

> > >> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93492#c2

> > >> >> If we

> > >> >>

> > >> >> * implement SHF_LINK_ORDER

> > >> >

> > >> >I am not sure if overloading (abusing?) SHF_LINK_ORDER is a good idea.

> > >>

> > >> It is an extension, but it is agreed by multiple parties on

> > >> https://groups.google.com/d/msg/generic-abi/_CbBM6T6WeM/eGF9A0AnAAAJ ,

> > >> including HP-UX and Solaris developers.

> > >>

> > >> >> * allow multiple sections with the same name ("unique")

> > >> >

> > >> >This is orthogonal to this.  I have a question on assembly syntax:

> > >> >

> > >> >https://sourceware.org/bugzilla/show_bug.cgi?id=25380#c1

> > >> >

> > >> >> * teach GCC to use SHF_LINK_ORDER and "unique" (see https://gcc.gnu.org/ml/gcc/2020-01/msg00067.html)

> > >> >>

> > >> >> An ad-hoc gc marking will be unnecessary.

> > >> >

> > >> >We need to scan relocations in _patchable_function_entries section for

> > >> >references to garbage collected sections.   We can either check section

> > >> >name or a SHF_XXX.  But I don't know if SHF_LINK_ORDER is a good

> > >> >approach.

> > >>

> > >> We don't need to if we use multiple __patchable_function_entries

> > >> sections. Multiple sections are a must due to

> > >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93195#c1 (COMDAT)

> > >>

> > >> > A symbol table entry with STB_LOCAL binding that is defined relative

> > >> > to one of a group's sections, and that is contained in a symbol table

> > >> > section that is not part of the group, must be discarded if the group

> > >> > members are discarded. References to this symbol table entry from

> > >> > outside the group are not allowed.

> > >>

> > >> The __patchable_function_entries creation logic I implemented in clang:

> > >>

> > >> foreach function foo

> > >>    find the first function label defined in foo's section, name it $associated

> > >>      ($associated can have 2 reasonable values, w/ or w/o -ffunction-sections)

> > >>    get or create an id for $associated, say, $unique

> > >>

> > >>    if foo is in a COMDAT named $comdat

> > >>      .section __patchable_function_entries,"awo",@progbits,$associated,comdat,$comdat,unique,$unique

> > >>    else

> > >>      .section __patchable_function_entries,"awo",@progbits,$associated,unique,$unique

> > >>

> > >> This approach uses feature requests I referenced in *direct* links of

> > >> https://gcc.gnu.org/ml/gcc/2020-01/msg00067.html plus

> > >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93492#c2 ,

> > >> and addresses all bugs I filed.

> > >>

> > >

> > >Here is a linker patch to issue an error to avoid corrupt

> > >linker output.

> > >

> > >--

> > >H.J.

> >

> > A typo in the description.

> >

> > - .section __patchable_function_entries,"awo",%progbits,_start

> > + .section __patchable_function_entries,"aw",%progbits

> >

> 

> Fixed.

> 


Now, gas supports the section flag 'o' in .section directive.  I will
submit a GCC patch to use it.  Here is the updated patch to issue an
error for garbage collection on __patchable_function_entries section
without linked-to section.

OK for master?

Thanks.

H.J.
---
__patchable_function_entries section is generated by a compiler with
-fpatchable-function-entry=XX.  The assembly code looks like this:

---
	.text
	.globl	_start
	.type	_start, %function
_start:
	.section __patchable_function_entries,"aw",%progbits
	.dc.a	.LPFE1
	.text
.LPFE1:
	.byte 0
---

But --gc-sections will silently remove __patchable_function_entries
section and generate corrupt result.  This patch disallows garbage
collection on __patchable_function_entries section without linked-to
section.

bfd/

 	PR ld/25490
	* elflink.c (_bfd_elf_gc_mark_extra_sections): Issue an error
	for garbage collection on __patchable_function_entries section
	without linked-to section.

ld/

 	PR ld/25490
	* testsuite/ld-elf/pr25490-1.d: New file.
	* testsuite/ld-elf/pr25490-1.s: Likewise.
---
 bfd/elflink.c                   | 7 +++++++
 ld/testsuite/ld-elf/pr25490-1.d | 3 +++
 ld/testsuite/ld-elf/pr25490-1.s | 9 +++++++++
 3 files changed, 19 insertions(+)
 create mode 100644 ld/testsuite/ld-elf/pr25490-1.d
 create mode 100644 ld/testsuite/ld-elf/pr25490-1.s

-- 
2.24.1

Comments

Alan Modra Feb. 7, 2020, 3:28 a.m. | #1
On Thu, Feb 06, 2020 at 06:23:28PM -0800, H.J. Lu wrote:
> bfd/

> 

>  	PR ld/25490

> 	* elflink.c (_bfd_elf_gc_mark_extra_sections): Issue an error

> 	for garbage collection on __patchable_function_entries section

> 	without linked-to section.

> 

> ld/

> 

>  	PR ld/25490

> 	* testsuite/ld-elf/pr25490-1.d: New file.

> 	* testsuite/ld-elf/pr25490-1.s: Likewise.


OK.

-- 
Alan Modra
Australia Development Lab, IBM

Patch

diff --git a/bfd/elflink.c b/bfd/elflink.c
index 30a572d32de..3add9f18bd7 100644
--- a/bfd/elflink.c
+++ b/bfd/elflink.c
@@ -13365,6 +13365,13 @@  _bfd_elf_gc_mark_extra_sections (struct bfd_link_info *info,
 	      && (isec->flags & SEC_DEBUGGING)
 	      && CONST_STRNEQ (isec->name, ".debug_line."))
 	    debug_frag_seen = TRUE;
+	  else if (strcmp (bfd_section_name (isec),
+			   "__patchable_function_entries") == 0
+		   && elf_linked_to_section (isec) == NULL)
+	      info->callbacks->einfo (_("%F%P: %pB(%pA): error: "
+					"need linked-to section "
+					"for --gc-sections\n"),
+				      isec->owner, isec);
 	}
 
       /* If no non-note alloc section in this file will be kept, then
diff --git a/ld/testsuite/ld-elf/pr25490-1.d b/ld/testsuite/ld-elf/pr25490-1.d
new file mode 100644
index 00000000000..3f582645899
--- /dev/null
+++ b/ld/testsuite/ld-elf/pr25490-1.d
@@ -0,0 +1,3 @@ 
+#ld: --gc-sections -e _start
+#target: [check_gc_sections_available]
+#error: .*\(__patchable_function_entries\): error: need linked-to section for --gc-sections
diff --git a/ld/testsuite/ld-elf/pr25490-1.s b/ld/testsuite/ld-elf/pr25490-1.s
new file mode 100644
index 00000000000..51ba1ea8014
--- /dev/null
+++ b/ld/testsuite/ld-elf/pr25490-1.s
@@ -0,0 +1,9 @@ 
+	.text
+	.globl	_start
+	.type	_start, %function
+_start:
+	.section	__patchable_function_entries,"aw",%progbits
+	.dc.a	.LPFE1
+	.text
+.LPFE1:
+	.byte 0