Thanks for your response.
On 2019-06-09, Joseph Koshy <[email protected]> wrote:
> On Sun, Jun 09, 2019 at 02:25:38AM -0700, Michael Forney wrote:
> m.f> 3. I also noticed that when elf_update recomputes
> m.f> the section layout, it leaves the section offset
> m.f> of SHT_NOBITS and SHT_NULL sections alone while
> m.f> the others are updated. elfutils sets the offset
> m.f> of the section based on the ending of the previous
> m.f> section. Does this offset have any meaning, or
> m.f> is it just arbitrary? Either way, maybe it makes
> m.f> sense to set it to the end of the previous section
> m.f> to avoid inheriting the value from the original
> m.f> section layout.
>
> Would you have a short example program that shows
> the difference in behavior of elfutils' libelf and
> elftoolchain's libelf?
>
> Per my understanding, for a section of type SHT_NULL,
> the rest of the fields in the section header are
> undefined. For a section of type SHT_NOBITS,
> the offset and size fields are defined to be a
> 'conceptual' offset and size of the section; these
> values would be set by the application that adds a
> SHT_NOBITS section to the object.
>
> When libelf is told to lay out an ELF object (i.e.,
> when the ELF_F_LAYOUT flag is not set on the Elf
> descriptor), then it isn't clear what the fields of a
> SHT_NOBITS section header would need to be set to.
I might be able to come up with an example later, but hopefully the
attached shell session explains the difference I'm seeing.
> m.f> 4. This is a bit unrelated to objtool,
> m.f> but I noticed that elfdefinitions.h contains
> m.f> several enums with values that are too big for
> m.f> int (specifically, EF_PPC_EMB, SHF_COMDEF,
> m.f> SHF_MIPS_STRING, SHF_EXCLUDE, SHF_MASKPROC,
> m.f> SHT_LOUSER, and SHT_HIUSER). The C standard says
> m.f> that all enumerator values must be representable as
> m.f> int, and have type int. Maybe it makes sense to use
> m.f> preprocessor defines for these instead? In addition
> m.f> to fixing ISO C conformance, this would also ensure
> m.f> that the enumerators have a consistent signedness
> m.f> (gcc will choose int for those less than 2^31,
> m.f> and unsigned for those that are greater or equal).
>
> I generally prefer using enumerations to preprocessor
> symbols.
Yes, generally I agree, but unfortunately the standard limits the
values to the range of int, so they aren't suitable for some purposes.
> As far as ISO C conformance goes, casting the literals
> defining these symbols to 'int' might suffice, since
> none of these values would need more than 32 bits
> for their representation; I will take a look.
Converting a value greater than INT_MAX to int is implementation
defined, but perhaps it would work as intended for all implementations
you care about. I also wonder if there are any cases where these large
constants are shifted right. If they were cast to int, the sign
extension could cause problems.
$ gcc -Wp,-MD,arch/x86/lib/.usercopy.o.d -nostdinc -isystem /lib/gcc/x86_64-linux-musl/8.2.0/include -I./arch/x86/include -I./arch/x86/include/generated -I./include -I./arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/kconfig.h -include ./include/linux/compiler_types.h -D__KERNEL__ -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE -Werror=implicit-function-declaration -Werror=implicit-int -Wno-format-security -std=gnu89 -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -m64 -falign-jumps=1 -falign-loops=1 -mno-80387 -mno-fp-ret-in-387 -mpreferred-stack-boundary=3 -mskip-rax-setup -mtune=generic -mno-red-zone -mcmodel=kernel -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_SSSE3=1 -DCONFIG_AS_AVX=1 -DCONFIG_AS_AVX2=1 -DCONFIG_AS_AVX512=1 -DCONFIG_AS_SHA1_NI=1 -DCONFIG_AS_SHA256_NI=1 -Wno-sign-compare -fno-asynchronous-unwind-tables -mindirect-branch=thunk-extern -mindirect-branch-register -fno-jump-tables -fno-delete-null-pointer-checks -Wno-frame-address -Wno-format-truncation -Wno-format-overflow -Wno-int-in-bool-context -O2 --param=allow-store-data-races=0 -Wframe-larger-than=2048 -fstack-protector-strong -Wno-unused-but-set-variable -Wno-unused-const-variable -fomit-frame-pointer -fno-var-tracking-assignments -Wdeclaration-after-statement -Wvla -Wno-pointer-sign -Wno-stringop-truncation -fno-strict-overflow -fno-merge-all-constants -fmerge-constants -fno-stack-check -fconserve-stack -Werror=date-time -Werror=incompatible-pointer-types -Werror=designated-init -fmacro-prefix-map=./= -Wno-packed-not-aligned -DKBUILD_BASENAME='usercopy' -DKBUILD_MODNAME='usercopy' -c -o arch/x86/lib/usercopy.o arch/x86/lib/usercopy.c
$ readelf -S arch/x86/lib/usercopy.o | tee orig.txt
There are 19 section headers, starting at offset 0x5f8:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS 0000000000000000 00000040
000000000000007b 0000000000000000 AX 0 0 16
[ 2] .rela.text RELA 0000000000000000 000003f0
0000000000000060 0000000000000018 I 17 1 8
[ 3] .data PROGBITS 0000000000000000 000000bb
0000000000000000 0000000000000000 WA 0 0 1
[ 4] .bss NOBITS 0000000000000000 000000bb
0000000000000000 0000000000000000 WA 0 0 1
[ 5] ___ksymtab_gpl+co PROGBITS 0000000000000000 000000c0
0000000000000008 0000000000000000 A 0 0 8
[ 6] .rela___ksymtab_g RELA 0000000000000000 00000450
0000000000000030 0000000000000018 I 17 5 8
[ 7] .altinstructions PROGBITS 0000000000000000 000000c8
000000000000001a 0000000000000000 A 0 0 1
[ 8] .rela.altinstruct RELA 0000000000000000 00000480
0000000000000060 0000000000000018 I 17 7 8
[ 9] .altinstr_replace PROGBITS 0000000000000000 000000e2
000000000000000a 0000000000000000 AX 0 0 1
[10] .rela.altinstr_re RELA 0000000000000000 000004e0
0000000000000030 0000000000000018 I 17 9 8
[11] .discard.addressa PROGBITS 0000000000000000 000000f0
0000000000000008 0000000000000000 WA 0 0 8
[12] .rela.discard.add RELA 0000000000000000 00000510
0000000000000018 0000000000000018 I 17 11 8
[13] __ksymtab_strings PROGBITS 0000000000000000 000000f8
0000000000000013 0000000000000000 A 0 0 1
[14] .comment PROGBITS 0000000000000000 0000010b
0000000000000012 0000000000000001 MS 0 0 1
[15] .note.GNU-stack PROGBITS 0000000000000000 0000011d
0000000000000000 0000000000000000 0 0 1
[16] .shstrtab STRTAB 0000000000000000 00000528
00000000000000ce 0000000000000000 0 0 1
[17] .symtab SYMTAB 0000000000000000 00000120
00000000000001f8 0000000000000018 18 15 8
[18] .strtab STRTAB 0000000000000000 00000318
00000000000000d6 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
l (large), p (processor specific)
$ cp arch/x86/lib/usercopy.o arch/x86/lib/usercopy.elfutils.o
$ ./tools/objtool/objtool.elfutils orc generate --no-fp --retpoline arch/x86/lib/usercopy.elfutils.o
$ readelf -S arch/x86/lib/usercopy.elfutils.o | tee elfutils.txt
There are 22 section headers, starting at offset 0x700:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS 0000000000000000 00000040
000000000000007b 0000000000000000 AX 0 0 16
[ 2] .rela.text RELA 0000000000000000 000000c0
0000000000000060 0000000000000018 I 17 1 8
[ 3] .data PROGBITS 0000000000000000 00000120
0000000000000000 0000000000000000 WA 0 0 1
[ 4] .bss NOBITS 0000000000000000 00000120
0000000000000000 0000000000000000 WA 0 0 1
[ 5] ___ksymtab_gpl+co PROGBITS 0000000000000000 00000120
0000000000000008 0000000000000000 A 0 0 8
[ 6] .rela___ksymtab_g RELA 0000000000000000 00000128
0000000000000030 0000000000000018 I 17 5 8
[ 7] .altinstructions PROGBITS 0000000000000000 00000158
000000000000001a 0000000000000000 A 0 0 1
[ 8] .rela.altinstruct RELA 0000000000000000 00000178
0000000000000060 0000000000000018 I 17 7 8
[ 9] .altinstr_replace PROGBITS 0000000000000000 000001d8
000000000000000a 0000000000000000 AX 0 0 1
[10] .rela.altinstr_re RELA 0000000000000000 000001e8
0000000000000030 0000000000000018 I 17 9 8
[11] .discard.addressa PROGBITS 0000000000000000 00000218
0000000000000008 0000000000000000 WA 0 0 8
[12] .rela.discard.add RELA 0000000000000000 00000220
0000000000000018 0000000000000018 I 17 11 8
[13] __ksymtab_strings PROGBITS 0000000000000000 00000238
0000000000000013 0000000000000000 A 0 0 1
[14] .comment PROGBITS 0000000000000000 0000024b
0000000000000012 0000000000000001 MS 0 0 1
[15] .note.GNU-stack PROGBITS 0000000000000000 0000025d
0000000000000000 0000000000000000 0 0 1
[16] .shstrtab STRTAB 0000000000000000 0000025d
00000000000000fd 0000000000000000 0 0 1
[17] .symtab SYMTAB 0000000000000000 00000360
00000000000001f8 0000000000000018 18 15 8
[18] .strtab STRTAB 0000000000000000 00000558
00000000000000d6 0000000000000000 0 0 1
[19] .orc_unwind_ip PROGBITS 0000000000000000 0000062e
0000000000000018 0000000000000004 A 0 0 1
[20] .rela.orc_unwind_ RELA 0000000000000000 00000648
0000000000000090 0000000000000018 I 17 19 8
[21] .orc_unwind PROGBITS 0000000000000000 000006d8
0000000000000024 0000000000000006 A 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
l (large), p (processor specific)
$ ./tools/objtool/objtool.elftoolchain orc generate --no-fp --retpoline arch/x86/lib/usercopy.elftoolchain.o
$ readelf -S arch/x86/lib/usercopy.elftoolchain.o > elftoolchain.txt
$ diff -u elfutils.txt elftoolchain.txt
--- elfutils.txt Sun Jun 9 19:54:52 2019
+++ elftoolchain.txt Sun Jun 9 19:54:44 2019
@@ -11,7 +11,7 @@
0000000000000060 0000000000000018 I 17 1 8
[ 3] .data PROGBITS 0000000000000000 00000120
0000000000000000 0000000000000000 WA 0 0 1
- [ 4] .bss NOBITS 0000000000000000 00000120
+ [ 4] .bss NOBITS 0000000000000000 000000bb
0000000000000000 0000000000000000 WA 0 0 1
[ 5] ___ksymtab_gpl+co PROGBITS 0000000000000000 00000120
0000000000000008 0000000000000000 A 0 0 8
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