On 8/19/25 2:50 AM, 赵佳炜 wrote:
libbpf/usdt can reject that if 'ti' is a static variable.
But how can we do that? It seems we cannot determine whether a variable is static during the preprocessing phase.
What I mean is after compilation and during libbpf processing usdt requests. At that time, libbpf can inspect the application binary and all usdt's. My above statement libbpf/usdt can reject that if 'ti' is a static variable. is not not precise, it should be libbpf/usdt can reject if there are ambiguity and provides some hints.
If the preprocessing phase means compilation, yes, we cannot do anything.
At 2025-08-19 01:35:04, "Yonghong Song" yonghong.song@linux.dev wrote:
On 8/17/25 6:43 AM, 赵佳炜 wrote:
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
Thanks for filing an issue on gcc and getting some feedback/suggestions from gcc community. Currently, libbpf/usdt does not suport format like '-1@ti(%rip)'. If we do intend to implement this. libbpf/usdt can reject that if 'ti' is a static variable. libbpf can provide some hints about how to make it work (see above [1] and [2]). Then, it would be user's reponsibility to change code so libbpf can support it.
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 >> [...]