Instead of a single source of symbols, now linker keeps a list of DSO (former Library) objects
with their own symbols map. That helps to process R_X86_64_COPY relocations correctly.
For example, if 'a.out' executable with dependencies ['libstdc++.so', 'libc.so'] is being loaded
and 'a.out' uses 'stdout' symbol from 'libc.so', its relocation process goes as follows:
- linker processes relocation entry 'stdout' of type R_X86_64_GLOB_DAT from 'libc.so',
- it goes through object list ['a.out', 'libstdc++.so', 'libc.so'] to find first object
that exports 'stdout' symbol. The symbol is in 'a.out' with the value e.g. '0x404070',
- linker sets 'stdout' symbol GOT entry in 'libc.so' to '0x404070',
....
- linker processes relocation entry 'stdout' of type R_X86_64_COPY from 'a.out',
- it goes through object list excluding 'a.out': ['libstdc++.so', 'libc.so']. The symbol is found in 'libc.so',
- linker copies the 'stdout' symbol content from 'libc.so' to memory at address '0x404070' (in 'a.out' object).
Objects are relocated in reverse order they were loaded. So in the example above, linker starts with relocating
'libc.so' and ends with 'a.out'. It is necessary e.g. when linking with 'libstdc++.so' - there are many
relocations which symbols are found in 'libstdc++.so', so they need to be resolved before their contents are
copied to 'a.out'. That also matches GNU ld.so behavior.
* load TLS segment for executable - while we can skip PT_LOAD for executable,
we still have to load TLS segment.
* set TCB address based on if elf is position independent
It seams that stdout of ld.so is not that much of an issue but actually
it unfortunately is. The major problem here is that sometimes programs
generate header files in stdout (./getmy_custom_headers > header.h) and
we need to keep that cleen. and this is very very popular in gcc.
This patch avoids collecting symbols, resolving relocs if they are
already done (usually for example libc.so during a dlopen for another
libfoo.so). This patch is purely for performance boost.
It is usually not optimal to load a library twice and for specifics,
it is **terrible** idea to load libc twice it was enough trouble
dealing with libc statically linked into ld.so. So What this patch does
it check for soname and if a library is already loaded it won't get
loaded again. Why soname ? because unfortunately some bins gets linked
againt libc.so while of their dependencies gets linked against
libc.so.6 while one is usually symbolic link for the other.
It is fact that ld.so has libc statially linked into it.
Normally we wouldn't need ld.so functionality once the program is
finalyl loaded, but with the next few patches, we will have dlopen which
will reuse the same ld.so functionality.
The problem is that it seams that huge part of the code is possible not
referntially transparent. That is, it is not impossible that some of the
functions have internals states. So when using the struct Linker that is
initialized by ld.so's copy of libc. we must access it using the same
copy even if both copies are identical.
For example in dlopen if you do linker.load_library(..). That would
segfault because it is using the function from libc not ld.so
So I don't truly undestand why should this be needed, but after long
hours of being stuck I thought maybe.. maybe that is the issue and
indeed it turned out to be so.
I attempted fixing this issue before at 43fbaf99. Although it did work,
it worked wrong, and it was just consistently working (but in wrong way)
until it didn't.
Since this is (hopefully) the real fix, I will try to explain exactly
what is going on.
This is explaination by example:
our TLS is memory of size 0x1000 starting at 0x7ffff6c50000,
but the real size is 0x000068 so we have padding stored at master.offset
= 0xf98
Now our symbol looks as follows
Offset Type Sym. Value Name
000000432b20 R_X86_64_DTPOFF64 0000000000000058 errno
The old code did 0x7ffff6c50000 + 0xf98 + 000000432b20 which is
obviosly overflowing the memory and wrong.
The right way 0x7ffff6c50000 + 0xf98 + 0000000000000058.
THe Tls base part and offset are added at __tls_get_addr function.
What is left is storing the 0x58 at the relocation address. The problem
is that we don't have 0x58, but we have (binary base + 0x58) in global
symbol table and binary base so what we store is the (binarybase + 0x58
- binary base).
I hope this does turn out to be wrong.
The problem here was that we alway added the base address, and we
assumed that all addresses we access are relative but this is not the
case in case of non pie binaries. The issue is that all addresses were
base+offset. so if we added the base again it will ofcourse generate
wrong address.
Jeremy Soller