On 13/02/23 16:22, Bartosz Golaszewski wrote:
On Mon, 13 Feb 2023 at 19:49, Adhemerval Zanella Netto adhemerval.zanella@linaro.org wrote:
On 13/02/23 12:49, Bartosz Golaszewski wrote:
Hey!
I'm the author and maintainer of libgpiod. I'm currently getting ready to do a new major release. After giving some exposure to the release candidate, I noticed that when using clang, I can't link against the C++ bindings, while it works just fine in GCC.
The tree in question is here: https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/log/
You can trigger the linking program by trying to build the C++ tests with clang like that:
CC=clang CXX=clang++ ./autogen.sh --enable-bindings-cxx --enable-tests && make -j16
You'll get the following error:
/usr/bin/ld: tests-chip.o:(.data+0x0): undefined reference to `typeinfo for gpiod::chip_closed' /usr/bin/ld: tests-line-request.o:(.data+0x0): undefined reference to `typeinfo for gpiod::request_released' /usr/bin/ld: .libs/gpiod-cxx-test: hidden symbol `_ZTIN5gpiod11chip_closedE' isn't defined /usr/bin/ld: final link failed: bad value
The typoinfo is missing for exception types that should be visible to users of the library.
The culprit is here: https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/tree/bindings/cxx/...
I added the GPIOD_CXX_BUILD macro in order to not re-export the visible symbols if any user of the library would include the gpiod.hpp header. When the library is being built, the symbols are visible, when someone includes the header, the symbols are hidden.
But the typeid of the symbol, for instance gpiod, will still be provided by shared library if I understood correctly:
libgpiod-llvm$ objdump -t ./bindings/cxx/tests/tests-chip.o 2>/dev/null| grep -w _ZTIN5gpiod11chip_closedE 0000000000000000 *UND* 0000000000000000 .hidden _ZTIN5gpiod11chip_closedE libgpiod-llvm$ objdump -t bindings/cxx/.libs/libgpiodcxx.so 2>/dev/null| grep _ZTIN5gpiod11chip_closedE 0000000000024b50 g O .data.rel.ro 0000000000000018 _ZTIN5gpiod11chip_closedE
However, it seems that GCC is not applying the hidden attribute on the typeid class:
libgpiod-gcc$ objdump -t ./bindings/cxx/tests/tests-chip.o 2>/dev/null| grep -w _ZTIN5gpiod11chip_closedE 0000000000000000 w O .data.rel.local.DW.ref._ZTIN5gpiod11chip_closedE 0000000000000008 .hidden DW.ref._ZTIN5gpiod11chip_closedE 0000000000000000 *UND* 0000000000000000 _ZTIN5gpiod11chip_closedE
When it creates create the vague linking weak symbol .data.rel.local.DW.ref._ZTIN5gpiod11chip_closedE.
I am not sure why GCC is being permissive here, in fact IMHO this is gcc issue. If I add the visibility explicitly using pragmas:
diff --git a/bindings/cxx/gpiodcxx/exception.hpp b/bindings/cxx/gpiodcxx/exception.hpp index 98b7bc4..24ae698 100644 --- a/bindings/cxx/gpiodcxx/exception.hpp +++ b/bindings/cxx/gpiodcxx/exception.hpp @@ -17,6 +17,8 @@
namespace gpiod {
+#pragma GCC visibility push(hidden)
/**
- @ingroup gpiod_cxx
- @{
@@ -25,7 +27,7 @@ namespace gpiod { /**
- @brief Exception thrown when an already closed chip is used.
*/ -class GPIOD_CXX_API chip_closed : public ::std::logic_error +class /*GPIOD_CXX_API*/ chip_closed : public ::std::logic_error { public:
@@ -64,6 +66,8 @@ public: virtual ~chip_closed(); };
+#pragma GCC visibility pop
/**
- @brief Exception thrown when an already released line request is used.
*/
I get an explicit linking error:
/usr/bin/ld: tests-chip.o:(.data.rel.local.DW.ref._ZTIN5gpiod11chip_closedE[DW.ref._ZTIN5gpiod11chip_closedE]+0x0): undefined reference to `typeinfo for gpiod::chip_closed'
Which is what I expect. So I suggest you to avoid adding the hidden visibility on tests because since there are not linking static, they should follow the default rules of ABI and hidden in this case does not really make much sense.
I'm not sure I understand this. The tests are linked dynamically - just like any other program would. IIUC: I build libgpiodcxx making the exception symbols visible, then if anyone else (a program linking against libgpiodcxx) includes the header, the symbol is hidden.
Hidden symbols linkage only works for TU within the same ELF object, for instance within a shared library or a binary itself. They are used, besides to trim the dynamic section, to also avoid symbol interposition through PLT.
So using hidden visibility for libgpiodcxx only makes sense if you other TU reference the exported ABI (for instance if another class from libgpiodcxx uses the chip_closed class). It does not make sense for external ELF objects to use visibility hidden for public ABIs because by ELF rules such symbols are not visible at static linking time.
For instance:
$ cat lib.cc #include <cstdio>
#include "lib.h"
Foo::Foo (void) { printf ("%s\n", __func__); }
Foo::~Foo (void) { printf ("%s\n", __func__); } $ cat lib.h class __attribute__ ((visibility ("hidden"))) Foo { public: Foo (void); ~Foo (void); }; $ cat main.cc #include "lib.h"
int main (int argc, char *argv[]) { Foo foo; return 0; }
$ g++ -std=gnu++17 -Wall -O2 -fpic -shared lib.cc -o libfoo.so $ g++ -std=gnu++17 -Wall -O2 main.cc -o main -L. -lfoo /usr/bin/ld: /tmp/ccntd0J3.o: in function `main': main.cc:(.text.startup+0x22): undefined reference to `Foo::Foo()' /usr/bin/ld: main.cc:(.text.startup+0x2a): undefined reference to `Foo::~Foo()' /usr/bin/ld: main: hidden symbol `_ZN3FooD1Ev' isn't defined /usr/bin/ld: final link failed: bad value collect2: error: ld returned 1 exit status
clang throws the exact same error, you need to use the default visibility for the exported ABI.
Now, the hidden visibility might help you to optimize the class usage for private methods.
For instance:
$ cat lib.h class __attribute__ ((visibility ("default"))) Foo { public: Foo (void); ~Foo (void);
void privateMethod(); }; $ cat lib.cc #include <cstdio>
#include "lib.h"
Foo::Foo (void) { privateMethod (); printf ("%s\n", __func__); }
Foo::~Foo (void) { printf ("%s\n", __func__); }
void Foo::privateMethod() { printf ("%s\n", __func__); } $ g++ -std=gnu++17 -Wall -O2 -fpic -shared lib.cc -o libfoo.so
This will always generate a PLT call for 'privateMethod' due ELF interposition rules:
$ objdump -d libfoo.so [...] 0000000000001170 <_ZN3FooC1Ev>: 1170: f3 0f 1e fa endbr64 1174: 48 83 ec 08 sub $0x8,%rsp 1178: e8 f3 fe ff ff call 1070 _ZN3Foo13privateMethodEv@plt 117d: 48 8d 3d 7d 0e 00 00 lea 0xe7d(%rip),%rdi # 2001 <_fini+0xe71> 1184: 48 83 c4 08 add $0x8,%rsp 1188: e9 d3 fe ff ff jmp 1060 puts@plt [...] $ objdump -R libfoo.so [...] 0000000000004020 R_X86_64_JUMP_SLOT _ZN3Foo13privateMethodEv@@Base [...]
The R_X86_64_JUMP_SLOT is the dynamic relocation resolved by dynamic loader. If you define that privateMethod has hidden visibility, compile know that external call are not supported and will optimize it away:
$ objdump -d libfoo.so: [...] 0000000000001150 <_ZN3FooD1Ev>: 1150: f3 0f 1e fa endbr64 1154: 48 83 ec 08 sub $0x8,%rsp 1158: 48 8d 3d af 0e 00 00 lea 0xeaf(%rip),%rdi # 200e <_fini+0xe8e> 115f: e8 ec fe ff ff call 1050 puts@plt 1164: 48 83 c4 08 add $0x8,%rsp 1168: c3 ret 1169: 90 nop 116a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1) [...]
You can see that it even allows the compiler to inline the privateMethod. Recent GCC and clang also provides another option, -fno-semantic-interposition, which might also improve PIC code generation [1].
[1] http://maskray.me/blog/2021-05-09-fno-semantic-interposition
On top of that - if I build the examples with clang, they build because they don't reference the symbols openly. But if I then make one of those exceptions fly out of libgpiod unconditionally, their typeid is correctly seen:
terminate called after throwing an instance of 'gpiod::request_released' what(): GPIO lines have been released Aborted
If on the other hand I add catch (const gpiod::request_released& ex) somewhere, I get the same linking error.
The same happens for any program that I'd build with -llgpiodcxx
Bart
If I make the symbols unconditionally visible here, clang starts to work but I have no idea why and would like to avoid re-exporting the symbols if I can.
I'm using the following version: Ubuntu clang version 15.0.6 Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /usr/bin
Host is: x86_64 GNU/Linux
It's not like gcc links fine but then fails to obtain typeid - I can catch exceptions coming out from libgpiod just fine in external apps linked using gcc and see their type.
Any hints?