v8dbg_* symbols not available when NodeJS is statically linked with V8

[ackalker]

When NodeJS is statically linked with libv8_base.a, which is the default, I've found that the v8dbg_* debug symbols generated by deps/v8/tools/gen-postmortem-metadata.py are not included in the node binaries. This makes debugging with GDB or writing DTrace or SystemTap probes more difficult than needed (see tools/genv8constants.py).

After some hunting around, I found that while debug-support.cc is being generated and compiled, the debug-support.o object file is never linked into the node binaries. This is because the linker decides that nothing in the node binary actually uses anything defined in debug-support.o, and skips the object file entirely.

I've found a surprisingly trivial fix: instead of a C++ file, create a header file instead, then #include it somewhere which is certainly going to get linked into node.

Any comments and suggestions are very welcome.

Tested with gdb and SystemTap

$ gdb out/Debug/node
[...]
(gdb) p v8dbg_SmiTagMask
$1 = 1

# stap -e 'probe process.function("main") { printf("%s: %s(%d)\n", pp(), execname(), pid()); printf("v8dbg_SmiTagMask = %d\n", @var("v8dbg_SmiTagMask@v8.cc")) }' -c out/Debug/node
process("/home/miki/vcs/git/node/out/Debug/node").function("main@../src/node_main.cc:64"): node(10908)
v8dbg_SmiTagMask = 1

[ackalker]

I would like to invite @bnoordhuis to have a look at this issue, since it affects v8's d8 and shell as well.

[ackalker]

Another alternative could be to use something like this: put all the debug variables in a self-initializing struct.

debug-support.h:

#ifndef _DEBUG_SUPPORT

#ifndef _V8VARS_DEF

typedef struct v8vars {
#define V8VAR(type, name, value)    type name;
#define _V8VARS_DEF

#else

} v8vars_t;
v8vars_t v8dbg = {

#define V8VAR(type, name, value)    value,
#define _DEBUG_SUPPORT

#endif

V8VAR(int, initialized, 0)
V8VAR(int, var1, 42)
V8VAR(char, var2, 'a')

#ifdef _DEBUG_SUPPORT

};

void v8dbg_init() {
    v8dbg.initialized = 1;
}

#else

#undef V8VAR
#include "debug-support.h"

#endif

#endif

main.cc:

#include <stdio.h>
#include <stdlib.h>

#include "debug-support.h"

int main(int argc, char *argv[]) {
    v8dbg_init();
    printf("Hello, world!\n");
    printf("The answer is: %d\n", v8dbg.var1);
    exit(0);
}

Though it may look hideous, it actually has a purpose: the preprocessor has to parse the header file twice, the v8dbg struct will be fully initialized at the end of the last pass.

This solution has several advantages:

• Although v8dbg is a struct, the generator script needs to loop over the list of variables only once.
• calling an initialize function which modifies a member of the struct will force the linker to include it in the final binary.
• the v8dbg_initialize() function makes a perfect target for an stap process.function probe to pick up the variables.

Again, any suggestions are very welcome.

[bnoordhuis]

@ackalker Is there a reason you closed this PR?

I've found a surprisingly trivial fix: instead of a C++ file, create a header file instead, then #include it somewhere which is certainly going to get linked into node.

The default symbol visibility is 'hidden' so I don't think that's going to work. Compiling with -fdata-sections -Wl,--gc-sections or -flto will almost certainly strip the symbols. You'd have to tag the generated symbols with __attribute__((visibility("default"))) to stop that from happening.

That said, we don't usually take V8 patches. I may make an exception in this case because it's obviously useful but I'd rather you send it upstream first.

[ackalker]

Thanks for your time :-)
The reason I closed the PR was that I thought that my first solution was too messy. I now have a tested, working patch.. I will open a new PR with pull request.