Hi, Yonghong. I've already filed an issue[1] in GCC community.
Accroding to the discussion, it's not a gcc bug but may be a systemtap bug. I don't know how to report this bug to systemtap, but I found that the libbpf/usdt have the same problem. I've filed an issue in libbpf/usdt repo[2].
I also have some ideas about it. I wrote it down in the issue[2] comment. May be we can discuss there.
[1]. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121569 [2]. https://github.com/libbpf/usdt/issues/13
At 2025-08-13 13:24:39, "Yonghong Song" yonghong.song@linux.dev wrote:
On 8/12/25 7:27 PM, 赵佳炜 wrote:
Sure.
The usdt_rip.c source code:
// the usdt_rip.c file #include <stddef.h> #include <stdint.h> #include "sdt.h" static volatile char ti = 0; int add(int a, int b) { return a + b; } int (*add_ptr)(int, int) = add; struct st{ int a; char b; }; volatile struct st t1 = {.a = 1, .b = 'a'}; static void __always_inline trigger_func() { STAP_PROBE4(usdt_rip, rip_global_var, ti, add_ptr, t1.b, ti); } int main() { trigger_func(); return 0; }The test/usdt_rip.c source code:
// The test/usdt_rip.c file static volatile char ti = 0;The compiler option is `gcc -O2 -g usdt_rip.c test/usdt_rip.c -o usdt_rip`
My compiler version:
$ gcc -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/13/lto-wrapper OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa OFFLOAD_TARGET_DEFAULT=1 Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 13.3.0-6ubuntu2~24.04' --with-bugurl=file:///usr/share/doc/gcc-13/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-13 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-libstdcxx-backtrace --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-13-fG75Ri/gcc-13-13.3.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-13-fG75Ri/gcc-13-13.3.0/debian/tmp-gcn/usr --enable-offload-defaulted --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=2 Thread model: posix Supported LTO compression algorithms: zlib zstd gcc version 13.3.0 (Ubuntu 13.3.0-6ubuntu2~24.04)Thanks for the code. I compiled with gcc14 compiler and the below is the asm code for 'main' function:
0000000000401020 <main>: 401020: 90 nop 401021: 31 c0 xor %eax,%eax 401023: c3 ret 401024: 66 2e 0f 1f 84 00 00 cs nopw 0x0(%rax,%rax,1) 40102b: 00 00 00 40102e: 66 90 xchg %ax,%ax
stapsdt note: Displaying notes found in: .note.stapsdt Owner Data size Description stapsdt 0x00000066 NT_STAPSDT (SystemTap probe descriptors) Provider: usdt_rip Name: rip_global_var Location: 0x0000000000401020, Base: 0x0000000000402010, Semaphore: 0x0000000000000000 Arguments: -1@ti(%rip) 8@add_ptr(%rip) -1@t1+4(%rip) -1@ti(%rip)
$ readelf -s usdt_rip | grep ti 4: 0000000000404019 1 OBJECT LOCAL DEFAULT 24 ti 14: 000000000040401a 1 OBJECT LOCAL DEFAULT 24 ti
So yet, it would be hard to find which 'ti' should be used to resolve usdt arguments.
I then tried with clang21. The 'main' function:
0000000000001140 <main>: 1140: 0f b6 05 f2 2e 00 00 movzbl 0x2ef2(%rip),%eax # 4039 <ti> 1147: 48 8b 0d da 2e 00 00 mov 0x2eda(%rip),%rcx # 4028 <add_ptr> 114e: 0f b6 15 df 2e 00 00 movzbl 0x2edf(%rip),%edx # 4034 <t1+0x4> 1155: 0f b6 35 dd 2e 00 00 movzbl 0x2edd(%rip),%esi # 4039 <ti> 115c: 88 44 24 f7 mov %al,-0x9(%rsp) 1160: 48 89 4c 24 f8 mov %rcx,-0x8(%rsp) 1165: 88 54 24 f6 mov %dl,-0xa(%rsp) 1169: 40 88 74 24 f5 mov %sil,-0xb(%rsp) 116e: 90 nop 116f: 31 c0 xor %eax,%eax 1171: c3 ret 1172: 66 90 xchg %ax,%ax
For this case, usdt should work properly. The following is related symbol table:
74: 0000000000004028 8 OBJECT GLOBAL DEFAULT 26 add_ptr 64: 0000000000004030 8 OBJECT GLOBAL DEFAULT 26 t1 50: 0000000000004039 1 OBJECT LOCAL DEFAULT 27 ti <=== for '# 4039 <ti>'.And for llvm21, only one 'ti' in the symbol table.
$ readelf -s usdt_rip | grep ti 50: 0000000000004039 1 OBJECT LOCAL DEFAULT 27 ti
The stapsdt note: Displaying notes found in: .note.stapsdt Owner Data size Description stapsdt 0x00000061 NT_STAPSDT (SystemTap probe descriptors) Provider: usdt_rip Name: rip_global_var Location: 0x000000000000116e, Base: 0x0000000000002004, Semaphore: 0x0000000000000000 Arguments: -1@-9(%rsp) 8@-8(%rsp) -1@-10(%rsp) -1@-11(%rsp)
It looks like clang optimized away the following: // The test/usdt_rip.c file static volatile char ti = 0; but gcc does not. But even if the above 'ti' is preserved for clang, clang generated code should still be fine since the argument is -1@-9(%rsp) 8@-8(%rsp) -1@-10(%rsp) -1@-11(%rsp).
I think you can file a bug for gcc community.
At 2025-08-13 00:11:45, "Yonghong Song" yonghong.song@linux.dev wrote:
On 8/12/25 12:02 AM, 赵佳炜 wrote:
Yes, I've tried that but it didn't help. FYI:
$ readelf -nsr usdt_rip
Relocation section '.rela.dyn' at offset 0x530 contains 9 entries: Offset Info Type Sym. Value Sym. Name + Addend 000000003df0 000000000008 R_X86_64_RELATIVE 1150 000000003df8 000000000008 R_X86_64_RELATIVE 1110 000000004008 000000000008 R_X86_64_RELATIVE 4008 000000004018 000000000008 R_X86_64_RELATIVE 1160 000000003fd8 000100000006 R_X86_64_GLOB_DAT 0000000000000000 __libc_start_main@GLIBC_2.34 + 0 000000003fe0 000200000006 R_X86_64_GLOB_DAT 0000000000000000 _ITM_deregisterTM[...] + 0 000000003fe8 000300000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0 000000003ff0 000400000006 R_X86_64_GLOB_DAT 0000000000000000 _ITM_registerTMCl[...] + 0 000000003ff8 000500000006 R_X86_64_GLOB_DAT 0000000000000000 __cxa_finalize@GLIBC_2.2.5 + 0
Symbol table '.dynsym' contains 6 entries: Num: Value Size Type Bind Vis Ndx Name 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND 1: 0000000000000000 0 FUNC GLOBAL DEFAULT UND _[...]@GLIBC_2.34 (2) 2: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterT[...] 3: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__ 4: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMC[...] 5: 0000000000000000 0 FUNC WEAK DEFAULT UND [...]@GLIBC_2.2.5 (3)
Symbol table '.symtab' contains 42 entries: Num: Value Size Type Bind Vis Ndx Name 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND 1: 0000000000000000 0 FILE LOCAL DEFAULT ABS Scrt1.o 2: 000000000000038c 32 OBJECT LOCAL DEFAULT 4 __abi_tag 3: 0000000000000000 0 FILE LOCAL DEFAULT ABS usdt_rip.c 4: 0000000000004021 1 OBJECT LOCAL DEFAULT 25 ti 5: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c 6: 00000000000010a0 0 FUNC LOCAL DEFAULT 14 deregister_tm_clones 7: 00000000000010d0 0 FUNC LOCAL DEFAULT 14 register_tm_clones 8: 0000000000001110 0 FUNC LOCAL DEFAULT 14 __do_global_dtors_aux 9: 0000000000004020 1 OBJECT LOCAL DEFAULT 25 completed.0 10: 0000000000003df8 0 OBJECT LOCAL DEFAULT 21 __do_global_dtor[...] 11: 0000000000001150 0 FUNC LOCAL DEFAULT 14 frame_dummy 12: 0000000000003df0 0 OBJECT LOCAL DEFAULT 20 __frame_dummy_in[...] 13: 0000000000000000 0 FILE LOCAL DEFAULT ABS damo.c 14: 0000000000004022 1 OBJECT LOCAL DEFAULT 25 ti 15: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c 16: 00000000000020d8 0 OBJECT LOCAL DEFAULT 19 __FRAME_END__ 17: 0000000000000000 0 FILE LOCAL DEFAULT ABS 18: 0000000000003e00 0 OBJECT LOCAL DEFAULT 22 _DYNAMIC 19: 0000000000002008 0 NOTYPE LOCAL DEFAULT 18 __GNU_EH_FRAME_HDR 20: 0000000000003fc0 0 OBJECT LOCAL DEFAULT 23 _GLOBAL_OFFSET_TABLE_ 21: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_mai[...] 22: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterT[...] 23: 0000000000004000 0 NOTYPE WEAK DEFAULT 24 data_start 24: 0000000000001160 8 FUNC GLOBAL DEFAULT 14 add 25: 0000000000004020 0 NOTYPE GLOBAL DEFAULT 24 _edata 26: 0000000000002004 1 NOTYPE WEAK HIDDEN 17 _.stapsdt.base 27: 0000000000004010 8 OBJECT GLOBAL DEFAULT 24 t1 28: 0000000000001168 0 FUNC GLOBAL HIDDEN 15 _fini 29: 0000000000004000 0 NOTYPE GLOBAL DEFAULT 24 __data_start 30: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__ 31: 0000000000004008 0 OBJECT GLOBAL HIDDEN 24 __dso_handle 32: 0000000000002000 4 OBJECT GLOBAL DEFAULT 16 _IO_stdin_used 33: 0000000000004028 0 NOTYPE GLOBAL DEFAULT 25 _end 34: 0000000000001070 38 FUNC GLOBAL DEFAULT 14 _start 35: 0000000000004020 0 NOTYPE GLOBAL DEFAULT 25 __bss_start 36: 0000000000001040 48 FUNC GLOBAL DEFAULT 14 main 37: 0000000000004018 8 OBJECT GLOBAL DEFAULT 24 add_ptr 38: 0000000000004020 0 OBJECT GLOBAL HIDDEN 24 __TMC_END__ 39: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMC[...] 40: 0000000000000000 0 FUNC WEAK DEFAULT UND __cxa_finalize@G[...] 41: 0000000000001000 0 FUNC GLOBAL HIDDEN 11 _init
Displaying notes found in: .note.gnu.property Owner Data size Description GNU 0x00000020 NT_GNU_PROPERTY_TYPE_0 Properties: x86 feature: IBT, SHSTK x86 ISA needed: x86-64-baseline
Displaying notes found in: .note.gnu.build-id Owner Data size Description GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring) Build ID: eb615daa575687cc44edc1d339b27890c12c27f1
Displaying notes found in: .note.ABI-tag Owner Data size Description GNU 0x00000010 NT_GNU_ABI_TAG (ABI version tag) OS: Linux, ABI: 3.2.0
Displaying notes found in: .note.stapsdt Owner Data size Description stapsdt 0x00000066 NT_STAPSDT (SystemTap probe descriptors) Provider: usdt_rip Name: rip_global_var Location: 0x0000000000001058, Base: 0x0000000000002004, Semaphore: 0x0000000000000000 Arguments: -1@ti(%rip) 8@add_ptr(%rip) -1@4+t1(%rip) -1@ti(%rip)
Could you share the complete source codes and compiler options which reproduce the above result?
At 2025-08-12 13:06:40, "Yonghong Song" yonghong.song@linux.dev wrote:
On 8/10/25 1:55 AM, 赵佳炜 wrote:
Hi Yonghong,
I found another issue where symbols can be duplicated, and I’m not sure how to tell them apart.
For example, I created two C files named usdt_rip.c and hello.c. Both define their own static ti variables, like:`static volatile char ti = 0;`.
After compiling, I obtained an ELF file usdt_rip whose .symtab contains the following entries:
$ readelf -s usdt_rip
Symbol table '.symtab' contains 42 entries: Num: Value Size Type Bind Vis Ndx Name 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND 1: 0000000000000000 0 FILE LOCAL DEFAULT ABS Scrt1.o 2: 000000000000038c 32 OBJECT LOCAL DEFAULT 4 __abi_tag 3: 0000000000000000 0 FILE LOCAL DEFAULT ABS usdt_rip.c 4: 0000000000004021 1 OBJECT LOCAL DEFAULT 25 ti 5: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c 6: 00000000000010a0 0 FUNC LOCAL DEFAULT 14 deregister_tm_clones 7: 00000000000010d0 0 FUNC LOCAL DEFAULT 14 register_tm_clones 8: 0000000000001110 0 FUNC LOCAL DEFAULT 14 __do_global_dtors_aux 9: 0000000000004020 1 OBJECT LOCAL DEFAULT 25 completed.0 10: 0000000000003df8 0 OBJECT LOCAL DEFAULT 21 __do_global_dtor[...] 11: 0000000000001150 0 FUNC LOCAL DEFAULT 14 frame_dummy 12: 0000000000003df0 0 OBJECT LOCAL DEFAULT 20 __frame_dummy_in[...] 13: 0000000000000000 0 FILE LOCAL DEFAULT ABS damo.c 14: 0000000000004022 1 OBJECT LOCAL DEFAULT 25 ti 15: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c 16: 00000000000020d8 0 OBJECT LOCAL DEFAULT 19 __FRAME_END__
As you can see, there are two ti variables in the .symtab section. Their values are very close, making them hard to distinguish.
I’m unsure how to handle this situation. Do you have any suggestions?
Did you check relocations? Relocaitons should be able to point exact which symbol.
Thanks, Jiawei Zhao
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