| Issue |
138584
|
| Summary |
[lld] Unsorted loadable program headers with `-Ttext=` argument
|
| Labels |
|
| Assignees |
|
| Reporter |
pskrgag
|
ELF spec say that program headers must be sorted by `p_vaddr`
> Loadable segment entries in the program header table appear in ascending order,
> sorted on the p_vaddr member
However it does not work if `-Ttext` argument is used.
Consider following example:
```c
int main(void)
{
while (1);
}
```
```bash
~/Documents/compiler_ws/lld
paskripkin > ~/Documents/git/llvm-project/build/bin/ld.lld -e main -Ttext=0x800000 test.o -o a.out
~/Documents/compiler_ws/lld
paskripkin > llvm-readelf -l a.out
Elf file type is EXEC (Executable file)
Entry point 0x800000
There are 5 program headers, starting at offset 64
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
PHDR 0x000040 0x0000000000200040 0x0000000000200040 0x000118 0x000118 R 0x8
LOAD 0x000000 0x0000000000200000 0x0000000000200000 0x000158 0x000158 R 0x1000
LOAD 0x001000 0x0000000000800000 0x0000000000800000 0x00000d 0x00000d R E 0x1000
LOAD 0x001010 0x0000000000801010 0x0000000000801010 0x00003c 0x00003c R 0x1000
GNU_STACK 0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW 0x0
Section to Segment mapping:
Segment Sections...
00
01
02 .text
03 .eh_frame
04
None .comment .symtab .shstrtab .strtab
```
Note that first loadable segment has a `p_vaddr` less than the second one. (still reproduces on current master c50cba6275271fba69be661b9ec0665b2be88dbc).
## Sort of analysis
The first loadable segment is the one that contains ELF header and program headers. It's added in `Writer<ELFT>::createPhdrs`.
```cpp
// Add the headers. We will remove them if they don't fit.
// In the other partitions the headers are ordinary sections, so they don't
// need to be added here.
if (isMain) {
load = addHdr(PT_LOAD, flags);
load->add(ctx.out.elfHeader.get());
load->add(ctx.out.programHeaders.get());
}
```
In normal case it gets merged into some of other segments because of the following code
```cpp
bool sameLMARegion =
load && !sec->lmaExpr && sec->lmaRegion == load->firstSec->lmaRegion;
if (load && sec != relroEnd &&
sec->memRegion == load->firstSec->memRegion &&
(sameLMARegion || load->lastSec == ctx.out.programHeaders.get()) &&
(ctx.script->hasSectionsCommand || sec->type == SHT_NOBITS ||
load->lastSec->type != SHT_NOBITS)) {
load->p_flags |= newFlags;
} else {
load = addHdr(PT_LOAD, newFlags);
flags = newFlags;
}
```
However if `-Ttext` is specified, predicate above may fail and new segment will be inserted. Then during `LinkerScript::assignAddresses`, first `PT_LOAD` segment gets assigned to `ctx.target->getImageBase()`, which may be far from the base of `text` (IIUC lld starts calculating from the base passed in `-T`).
## Note
This is not a synthetic example. It was observed in real life with very strict elf loader.
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