[SR-13165] Is splitting ImageInspectionELF.cpp into swiftImageInspectionShared necessary?

[swift-ci]

Previous ID	SR-13165
Radar	None
Original Reporter	3405691582 (JIRA User)
Type	Task
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s
Labels	Task
Assignee	@spevans
Priority	Medium

md5: 690e563998a653a367ea95c34a707ee2

Issue Description:

stdlib/public/runtime/CMakeLists.txt splits ImageInspectionELF.cpp out from swift_runtime_sources into its own library, swiftImageInspectionShared. There appear to still be references from Errors.cpp in swift_runtime_sources to lookupSymbol in ImageInspectionELF.cpp, which means when SWIFT_BUILD_STATIC_STDLIB, if swiftImageInspectionShared is not referred to, complaints about undefined symbols when lazy binding will occur.

This additional error output fouls up the expectation in a number of unit tests, see pr #32736. This only occurs when SWIFT_BUILD_STATIC_STDLIB. (Specifically, the reference in Errors.cpp is only when SWIFT_SUPPORTS_BACKTRACE_REPORTING too.)

Is making this split truly necessary? I suspect it may not be, but I'd have to do some testing on Linux machines to see what the effect would be. See pr #5394, SR-648.

[swift-ci]

Comment by 3405691582 (JIRA)

cc @spevans

[spevans]

The reason for the split into swiftImageInspectionShared and swiftImageInspectionStatic is to support -static-stdlib and -static-executable

When building with -static-stdlib the exectuable is a dynamic executable that links in the .a versions of libswiftCore libswifDispatch etc but to implement swift::lookupSymbol it uses dladdr and links in -ldl as is common for standard ELF shared executables on Linux.

When building a fully static executable using -static-executable the libdl functions cant be used so swift::lookupSymbol is implemented by directly parsing the ELF sections in the static binary.

So because there are 2 different implementations of swift::lookupSymbol there has to be 2 different libswiftImageInspection{Shared,Static}.a libraries.

Note that -static-executable is currently broken, this was due to CMakefile.txt changes made a while ago, there is a PR to fix it #29039 but it ended up stagnating.

[spevans]

Note swift::lookupSymbol is used for stacktraces but dladdr wont work in static binaries

Example:

$ cat dladdr-test.c 
#include <stdio.h>
#define  __USE_GNU 1
#include <dlfcn.h>

int main() {
        Dl_info info = {};
        if (dladdr(main, &info)) {
                printf("fname:\t%s\n", info.dli_fname);
                printf("base:\t%p\n", info.dli_fbase);
                printf("sname:\t%s\ns", info.dli_sname);
                printf("saddr:\t%p\n", info.dli_saddr);
        } else {
                puts("Cant dladdr() main");
        }
        return 0;
}

# Ubuntu 18.04
$ clang -Wall -o dynamic-dladdr dladdr-test.c -ldl
$ ./dynamic-dladdr 
fname:  ./dynamic-dladdr
base:   0x400000
sname:  (null)
ssaddr: (nil)

$ clang -Wall -static -o static-dladdr dladdr-test.c -ldl
$ ./static-dladdr 
Cant dladdr() main

# OpenBSD 6.6

$ clang -Wall -o dynamic-dladdr dladdr-test.c
$ ./dynamic-dladdr                                                                                                                
fname:  ./dynamic-dladdr
base:   0x160e3000
sname:  (null)
ssaddr: 0x0

$ clang -Wall -static -o static-dladdr dladdr-test.c  
$ ./static-dladdr                                                                                                                 
Wrong dl symbols!
fname:  (null)
base:   0x0
sname:  (null)
ssaddr: 0x0

[spevans]

There is a new version of my PR @ #34180 that removes the split at the cost of swift::lookupSymbol not working in static executables. However that code was very Linux specific in how it found the loaded ELF exectuable and would not have work on OpenBSD anyway.

[spevans]

The split has been removed now and there is no separate swiftImageInspectionShared anymore.

[swift-ci]

Comment by 3405691582 (JIRA)

Thank you!