Currently, obj2yaml doesn't emit the offset of program headers, leaving
it to yaml2obj to calculate offsets based on `FirstSec` and `LastSec`.
This causes an obj2yaml->yaml2obj round trip to often produce an ELF
file that is not equivalent to the original, especially since it seems
common to have program headers at offset 0 whose first section starts at
a higher address. Besides being non-equivalent, the produced ELF files
also do not seem to work propery and readelf complains about them.
Taking a simple hello world program in C, compiled using either GCC or
Clang, the original ELF file has the following program headers (only
showing some relevant ones):
```
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
PHDR 0x0000000000000040 0x0000000000000040 0x0000000000000040
0x00000000000002d8 0x00000000000002d8 R 0x8
INTERP 0x0000000000000318 0x0000000000000318 0x0000000000000318
0x000000000000001c 0x000000000000001c R 0x1
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
LOAD 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000630 0x0000000000000630 R 0x1000
LOAD 0x0000000000001000 0x0000000000001000 0x0000000000001000
0x0000000000000161 0x0000000000000161 R E 0x1000
...
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .note.gnu.property .note.gnu.build-id .note.ABI-tag .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt
03 .init .plt .text .fini
...
```
While this is the result of an obj2yaml->yaml2obj round trip:
```
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
PHDR 0x0000000000000000 0x0000000000000040 0x0000000000000040
0x0000000000000000 0x0000000000000000 R 0x8
readelf: Error: the PHDR segment is not covered by a LOAD segment
INTERP 0x0000000000000318 0x0000000000000318 0x0000000000000318
0x000000000000001c 0x000000000000001c R 0x1
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
LOAD 0x0000000000000318 0x0000000000000000 0x0000000000000000
0x0000000000000318 0x0000000000000318 R 0x1000
LOAD 0x0000000000001000 0x0000000000001000 0x0000000000001000
0x0000000000000161 0x0000000000000161 R E 0x1000
...
Section to Segment mapping:
Segment Sections...
00
01 .interp
02
03 .init .plt .text .fini
...
```
Note that the offset of segment 2 changed from 0x0 to 0x318. This has
two effects:
- readelf complains "Error: the PHDR segment is not covered by a LOAD
segment" since PHDR was originally covered by segment 2 but not
anymore;
- Segment 2 effectively became empty according to the section to segment
mapping.
I addition to these, the output doesn't correctly execute anymore,
crashing with a "SIGSEGV (Address boundary error)".
This patch fixes the difference in program header layout after a round
trip by explicitly emitting offsets.
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D145555
8545093715