Summary:
This patch adds support for the general dynamic TLS access model for X86_64 (see www.akkadia.org/drepper/tls.pdf).
To properly support TLS, the patch also changes the __tls_get_addr atom to be a shared library atom instead of a regularly defined atom (the previous lld approach). This closely models the reality of a function that will be resolved at runtime by the dynamic linker and loader itself (ld.so). I was tempted to force LLD to link against ld.so itself to resolve these symbols, but since GNU ld does not need the ld.so library to resolve this symbol, I decided to mimic its behavior and keep hardwired a definition of __tls_get_addr in the lld code.
This patch also moves some important logic that previously was only available to the MIPS lld backend to be used to all ELF backends. This logic, which now lives in the DefaultLayout class, will monitor which external (shared lib) symbols are really imported by the current module and will only populate the dynamic symbol table with used symbols, as opposed to the previous approach of dumping all shared lib symbols in the dynamic symbol table. This is important to this patch to avoid __tls_get_addr from getting injected into all dynamic symbol tables.
By solving the previous problem of always adding __tls_get_addr, now the produced symbol tables are slightly smaller. But this impacted several tests that relied on hardwired/predefined sizes of the symbol table, requiring this patch to update such tests.
Test Plan: Added a LIT test case that exercises a simple use case of TLS variable in a shared library.
Reviewers: ruiu, rafael, Bigcheese, shankarke
Reviewed By: Bigcheese, shankarke
Subscribers: emaste, shankarke, joerg, kledzik, mcrosier, llvm-commits
Projects: #lld
Differential Revision: http://reviews.llvm.org/D5505
llvm-svn: 218633
Patch from Rafael Auler!
When a shared lib has an undefined symbol that is defined in a regular object
(the program), the final executable must export this symbol in the dynamic
symbol table. However, in the current logic, lld only puts the symbol in the
dynamic symbol table if the symbol is weak. This patch fixes lld to put the
symbol in the dynamic symbol table regardless if it is weak or not.
This caused a problem in FreeBSD10, whose programs link against a crt1.o
that defines the symbol __progname, which is, in turn, undefined in libc.so.7
and will only be resolved in runtime.
http://reviews.llvm.org/D5424
llvm-svn: 218259
The existing system linkers on Darwin and Linux are called "ld". We'd like to
eventually drop in lld as "ld" and have it just work. But lld is a universal
linker that requires the first option to be -flavor to know which command line
mode to emulate (gnu or darwin).
This change tests if argv[0] is "ld" and if so, if the tool was built on MacOSX
then assume the darwin flavor otherwise the gnu flavor. There are two test
cases which copy lld to "ld" and then run it. One for darwin and one for linux.
llvm-svn: 217566
defined in a shared library.
Now LLD does not export a strong defined symbol if it coalesces away a
weak symbol defined in a shared library. This bug affects all ELF
architectures and leads to segfault:
% cat foo.c
extern int __attribute__((weak)) flag;
int foo() { return flag; }
% cat main.c
int flag = 1;
int foo();
int main() { return foo() == 1 ? 0 : -1; }
% clang -c -fPIC foo.c main.c
% lld -flavor gnu -target x86_64 -shared -o libfoo.so ... foo.o
% lld -flavor gnu -target x86_64 -o a.out ... main.o libfoo.so
% ./a.out
Segmentation fault
The problem is caused by the fact that we lose all information about
coalesced symbols after the `Resolver::resolve()` method is finished.
The patch solves the problem by overriding the
`LinkingContext::notifySymbolTableCoalesce()` method and saving names
of coalesced symbols. Later in the `buildDynamicSymbolTable()` routine
we use this information to export these symbols.
llvm-svn: 217363
When a file is not found, produce a proper error message. The previous error
message produced a file format error, which made me wonder for a while why
there is a file format error, but essentially the file was not found.
This fixes the problem by producing a proper error message.
llvm-svn: 217359
By default linker would not create a separate segment to hold read only data.
This option overrides that behavior by creating the a separate read only segment
for read only data.
llvm-svn: 217358
This patch adds the initial ELF/AArch64 support to lld. Only a basic "Hello
World" app has been successfully tested for both dynamic and static compiling.
Differential Revision: http://reviews.llvm.org/D4778
Patch by Daniel Stewart <stewartd@codeaurora.org>!
llvm-svn: 215544
The commit of the .ent/.end implementation will change the result of the
relocation evaluations (because of a new section and additional relocations)
which will lead to a failure if the .ent/.end directives are present in this
test.
We don't really need the .ent/.end directives in this test so let's just remove
them to preserve the current output.
llvm-svn: 215534
There are two forms of `-l` prefixed expression:
* -l<libname>
* -l:<filename>
In the first case a linker should construct a full library name
`lib + libname + .[so|a]` and search this library as usual. In the second case
a linker should use the `<filename>` as is and search this file through library
search directories.
The patch reviewed by Shankar Easwaran.
llvm-svn: 213077
dynamic symbol table populating and DT_NEEDED tag creation.
The `isDynSymEntryRequired` function returns true if the specified shared
library atom requires a dynamic symbol table entry. The `isNeededTagRequired`
function returns true if we need to create DT_NEEDED tag for the shared
library defined specified shared atom.
By default the both functions return true. So there is no functional changes
for all targets except MIPS. Probably we need to spread the same modifications
on other ELF targets but I want to implement and fully tested complete set of
changes for MIPS target first.
For MIPS we create a dynamic symbol table entry for a shared library atom iif
this atom is referenced by a regular defined atom. For example, if library L1
defines symbol T1, library L2 defines symbol T2 and uses symbol T1
and executable file E1 uses symbol T2 but does not use symbol T1 we create
an entry in the E1 dynamic symbol table for symbol T2 and do not create
an entry for T1.
The patch creates DT_NEEDED tags for shared libraries contain shared library
atoms which a) referenced by regular defined atoms; b) have corresponding
copy dynamic relocations (R_MIPS_COPY).
Now the patch does not take in account --as-needed / --no-as-needed command
line options. So it is too restrictive and create DT_NEEDED tags for really
needed shared libraries only. I plan to fix that by subsequent patches.
llvm-svn: 211674
Rafael Auler