RFR: 8376269: Mixed jstack cannot find function in vDSO [v4]

[Yasumasa Suenaga]

On Wed, 4 Feb 2026 11:56:15 GMT, Kevin Walls <kevinw at openjdk.org> wrote:

>> Yasumasa Suenaga has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:
>> 
>>  - Update testcase to skip if kernel debuginfo is not available
>>  - Merge branch 'master' into JDK-8376269
>>  - Add new function to handle vDSO
>>  - Remove unneeded change
>>  - Remove 32 bit support
>>  - Update src/jdk.hotspot.agent/linux/native/libsaproc/ps_core.c
>>    
>>    Co-authored-by: Chris Plummer <chris.plummer at oracle.com>
>>  - Update src/jdk.hotspot.agent/linux/native/libsaproc/ps_core.c
>>    
>>    Co-authored-by: Chris Plummer <chris.plummer at oracle.com>
>>  - 8376269: Mixed jstack cannot find function in vDSO
>
> src/jdk.hotspot.agent/linux/native/libsaproc/ps_core.c line 732:
> 
>> 730: 
>> 731:       if (lib_name[0] != '\0') { // ignore empty lib names
>> 732:          if (strcmp("linux-vdso.so.1", lib_name) == 0 ||
> 
> Could we check this lib is at the VDSO address, so we know for sure it's the one and only system vdso?
>  link_map_addr + LINK_MAP_ADDR_OFFSET == ps_prochandle->core->vdso_addr
> Could do that instead of the name check, or in addition.

I think it is better to compare with `lib_base` like this:

         if (lib_base == ph->core->vdso_addr) {
           lib_fd = handle_vdso(ph, lib_name, sizeof(lib_name));
         } else {

However it is difficult because `lib_base` is calculated in subsequent code with program header in ELF (in `find_base_address`).

Maybe we can use `lib_base_diff` at here because debug log says virtual address for vDSO is same with vaddr in `l_addr` in `struct link_map` as following, but I'm not sure.

libsaproc DEBUG: first link map is at 0x7fa08970d2f0
libsaproc DEBUG: replace vDSO: linux-vdso.so.1 -> /lib/modules/6.18.6-200.fc43.x86_64/vdso/vdso64.so
libsaproc DEBUG: reading library /lib/modules/6.18.6-200.fc43.x86_64/vdso/vdso64.so @ 0x7fa0896d2000 [ 0x7fa0896d2000 ]
libsaproc DEBUG: overwrote with new address mapping (memsz 8192 -> 8192)
libsaproc DEBUG: /lib/modules/6.18.6-200.fc43.x86_64/vdso/vdso64.so [0] 0x7fa0896d2000-0x7fa0896d4000: base = 0x7fa0896d2000, vaddr = 0x0, memsz = 0x1883, filesz = 0x1883

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PR Review Comment: https://git.openjdk.org/jdk/pull/29400#discussion_r2766575815