commit 6edf175d10694eb1b410bc2ac166ed77c35944ef
author Fangrui Song <maskray@google.com> Mon Sep 16 07:04:16 2019 +0000
committer Fangrui Song <maskray@google.com> Mon Sep 16 07:04:16 2019 +0000
tree a5133f74358132775dce0e908f0278457e27aa71
parent 68496f45341840bfe4061213a6c6a1e90d2df850

[ELF] Map the ELF header at imageBase

If there is no readonly section, we map:

* The ELF header at imageBase+maxPageSize
* Program headers at imageBase+maxPageSize+sizeof(Ehdr)
* The first section .text at imageBase+maxPageSize+sizeof(Ehdr)+sizeof(program headers)

Due to the interaction between Writer<ELFT>::fixSectionAlignments and
LinkerScript::allocateHeaders,
`alignDown(p_vaddr(R PT_LOAD)) = alignDown(p_vaddr(RX PT_LOAD))`.
The RX PT_LOAD will override the R PT_LOAD at runtime, which is not ideal:

```
// PHDR at 0x401034, should be 0x400034
  PHDR           0x000034 0x00401034 0x00401034 0x000a0 0x000a0 R   0x4
// R PT_LOAD contains just Ehdr and program headers.
// At 0x401000, should be 0x400000
  LOAD           0x000000 0x00401000 0x00401000 0x000d4 0x000d4 R   0x1000
  LOAD           0x0000d4 0x004010d4 0x004010d4 0x00001 0x00001 R E 0x1000
```

* createPhdrs allocates the headers to the R PT_LOAD.
* fixSectionAlignments assigns `imageBase+maxPageSize+sizeof(Ehdr)+sizeof(program headers)` (formula: `alignTo(dot, maxPageSize) + dot % config->maxPageSize`) to addrExpr of .text
* allocateHeaders computes the minimum address among SHF_ALLOC sections, i.e. addr(.text)
* allocateHeaders sets address of ELF header to `addr(.text)-sizeof(Ehdr)-sizeof(program headers) = imageBase+maxPageSize`

The main observation is that when the SECTIONS command is not used, we
don't have to call allocateHeaders. This requires an assumption that
the presence of PT_PHDR and addresses of headers can be decided
regardless of address information.

This may seem natural because dot is not manipulated by a linker script.
The other thing is that we have to drop the special rule for -T<section>
in `getInitialDot`. If -Ttext is smaller than the image base, the headers
will not be allocated with the old behavior (allocateHeaders is called)
but always allocated with the new behavior.

The behavior change is not a problem. Whether and where headers are
allocated can vary among linkers, or ld.bfd across different versions
(--enable-separate-code or not). It is thus advised to use a linker
script with the PHDRS command to have a consistent behavior across
linkers. If PT_PHDR is needed, an explicit --image-base can be a simpler
alternative.

Differential Revision: https://reviews.llvm.org/D67325

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@371957 91177308-0d34-0410-b5e6-96231b3b80d8