https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77609
Bug ID: 77609
Summary: __attribute__((section(".note.foo"))) forces
SHT_PROGBITS though the assembler would use SHT_NOTE
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: roland at gnu dot org
Target Milestone: ---
When __attribute__((section("name"))) is used, GCC insists on setting the
section type explicitly. For various special section names, the assembler
would use a special type, but GCC doesn't know about those rules and forces
SHT_PROGBITS instead. An example is section names starting with ".note",
which the assembler gives type SHT_NOTE but GCC fails to. This makes it
impossible (without heinous shenanigans) to use an initialized C variable
declaration to emit an (allocated) ELF note.
GCC already has special cases for ".init_array", ".fini_array", and
".preinit_array", but that is a small subset of the full set of names (and
name patterns) for which the assembler uses a special type. The existing
special cases already rely on the assembler to choose the actual type. The
default type of SHT_PROGBITS that GCC forces on all others is already the
assembler's default for sections whose name is not special. So just
letting the assembler choose in all cases does the right thing now and will
do so in the future if any new types or special name patterns are added to
the assembler.
I'll attach a patch that does this.
Test case:
$ cat note.c
#define VENDOR "foobar"
struct mynote {
struct Elf32_Nhdr {
unsigned int n_namesz, n_descsz, n_type;
} hdr;
char name[sizeof(VENDOR)];
_Alignas(4) struct mynote_payload {
short int x;
} payload;
};
__attribute__((used, section(".note.foo")))
static const struct mynote foonote = {
.hdr = {
.n_namesz = sizeof(VENDOR),
.n_descsz = sizeof(struct mynote_payload),
.n_type = 17,
},
.name = VENDOR,
.payload = {
.x = 23,
},
};
$ ./gcc/xgcc -Bgcc/ -c -save-temps ../../gcc/note.c
$ cat note.s
.file "note.c"
==> .section .note.foo,"a",@progbits
.align 16
.type foonote, @object
.size foonote, 24
foonote:
.long 7
.long 2
.long 17
.string "foobar"
.zero 1
.value 23
.zero 2
.ident "GCC: (GNU) 7.0.0 20160915 (experimental)"
.section .note.GNU-stack,"",@progbits
$ readelf -WSn note.o
There are 10 section headers, starting at offset 0xd8:
Section Headers:
[Nr] Name Type Address Off Size
ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000
00 0 0 0
[ 1] .text PROGBITS 0000000000000000 000040 000000
00 AX 0 0 1
[ 2] .data PROGBITS 0000000000000000 000040 000000
00 WA 0 0 1
[ 3] .bss NOBITS 0000000000000000 000040 000000
00 WA 0 0 1
==> [ 4] .note.foo PROGBITS 0000000000000000 000040 000018
00 A 0 0 16
[ 5] .comment PROGBITS 0000000000000000 000058 00002a
01 MS 0 0 1
[ 6] .note.GNU-stack PROGBITS 0000000000000000 000082 000000
00 0 0 1
[ 7] .shstrtab STRTAB 0000000000000000 000082 00004f
00 0 0 1
[ 8] .symtab SYMTAB 0000000000000000 000358 0000d8
18 9 9 8
[ 9] .strtab STRTAB 0000000000000000 000430 000010
00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
I (info), L (link order), G (group), T (TLS), E (exclude), x
(unknown)
O (extra OS processing required) o (OS specific), p (processor
specific)
$
After my fix:
$ ./gcc/xgcc -Bgcc/ -c -save-temps ../../gcc/note.c
$ cat note.s
.file "note.c"
==> .section .note.foo,"a"
.align 16
.type foonote, @object
.size foonote, 24
foonote:
.long 7
.long 2
.long 17
.string "foobar"
.zero 1
.value 23
.zero 2
.ident "GCC: (GNU) 7.0.0 20160915 (experimental)"
.section .note.GNU-stack,"",@progbits
$ readelf -WSn note.o
There are 10 section headers, starting at offset 0xd8:
Section Headers:
[Nr] Name Type Address Off Size
ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000
00 0 0 0
[ 1] .text PROGBITS 0000000000000000 000040 000000
00 AX 0 0 1
[ 2] .data PROGBITS 0000000000000000 000040 000000
00 WA 0 0 1
[ 3] .bss NOBITS 0000000000000000 000040 000000
00 WA 0 0 1
==> [ 4] .note.foo NOTE 0000000000000000 000040 000018
00 A 0 0 16
[ 5] .comment PROGBITS 0000000000000000 000058 00002a
01 MS 0 0 1
[ 6] .note.GNU-stack PROGBITS 0000000000000000 000082 000000
00 0 0 1
[ 7] .shstrtab STRTAB 0000000000000000 000082 00004f
00 0 0 1
[ 8] .symtab SYMTAB 0000000000000000 000358 0000d8
18 9 9 8
[ 9] .strtab STRTAB 0000000000000000 000430 000010
00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
I (info), L (link order), G (group), T (TLS), E (exclude), x
(unknown)
O (extra OS processing required) o (OS specific), p (processor
specific)
Displaying notes found at file offset 0x00000040 with length
0x00000018:
Owner Data size Description
==> foobar 0x00000002 Unknown note type: (0x00000011)
$
