Re: [PATCH v2] libbpf: Improve version handling when attaching uprobe

[Yonghong Song <yhs@meta.com>]

On 4/27/23 12:19 PM, Espen Grindhaug wrote:
> On Wed, Apr 26, 2023 at 02:47:27PM -0700, Yonghong Song wrote:
>>
>>
>> On 4/23/23 11:55 AM, Espen Grindhaug wrote:
>>> This change fixes the handling of versions in elf_find_func_offset.
>>> In the previous implementation, we incorrectly assumed that the
>>
>> Could you give more explanation/example in the commit message
>> what does 'incorrectly' mean here? In which situations the
>> current libbpf implementation will not be correct?
>>
> 
> How about something like this?
> 
> 
> libbpf: Improve version handling when attaching uprobe
> 
> This change fixes the handling of versions in elf_find_func_offset.
> 
> For example, let's assume we are trying to attach an uprobe to pthread_create in
> glibc. Prior to this commit, it would fail with an error message saying 'elf:
> ambiguous match [...]', this is because there are two entries in the symbol
> table with that name.
> 
> $ nm -D /lib/x86_64-linux-gnu/libc.so.6 | grep pthread_create
> 0000000000094cc0 T pthread_create@GLIBC_2.2.5
> 0000000000094cc0 T pthread_create@@GLIBC_2.34
> 
> So we go ahead and modify our code to attach to 'pthread_create@@GLIBC_2.34',
> and this also fails, but this time with the error 'elf: failed to find symbol
> [...]'. This fails because we incorrectly assumed that the version information
> would be present in the string found in the string table, but there is only the
> string 'pthread_create'.

I tried one example with my centos8 libpthread library.

$ llvm-readelf -s /lib64/libc-2.28.so | grep pthread_cond_signal
     39: 0000000000095f70    43 FUNC    GLOBAL DEFAULT    14 
pthread_cond_signal@@GLIBC_2.3.2
     40: 0000000000096250    43 FUNC    GLOBAL DEFAULT    14 
pthread_cond_signal@GLIBC_2.2.5
   3160: 0000000000096250    43 FUNC    LOCAL  DEFAULT    14 
__pthread_cond_signal_2_0
   3589: 0000000000095f70    43 FUNC    LOCAL  DEFAULT    14 
__pthread_cond_signal
   5522: 0000000000095f70    43 FUNC    GLOBAL DEFAULT    14 
pthread_cond_signal@@GLIBC_2.3.2
   5545: 0000000000096250    43 FUNC    GLOBAL DEFAULT    14 
pthread_cond_signal@GLIBC_2.2.5
$ nm -D /lib64/libc-2.28.so | grep pthread_cond_signal
0000000000095f70 T pthread_cond_signal@@GLIBC_2.3.2
0000000000096250 T pthread_cond_signal@GLIBC_2.2.5
$

Note that two pthread_cond_signal functions have different addresses,
which is expected as they implemented for different versions.

But in your case,
 > $ nm -D /lib/x86_64-linux-gnu/libc.so.6 | grep pthread_create
 > 0000000000094cc0 T pthread_create@GLIBC_2.2.5
 > 0000000000094cc0 T pthread_create@@GLIBC_2.34

Two functions have the same address which is very weird and I suspect
some issues here at least needs some investigation.

Second, for the symbol table, the following is ELF encoding,

typedef struct {
         Elf64_Word      st_name;
         unsigned char   st_info;
         unsigned char   st_other;
         Elf64_Half      st_shndx;
         Elf64_Addr      st_value;
         Elf64_Xword     st_size;
} Elf64_Sym;

where
st_name

     An index into the object file's symbol string table, which holds 
the character representations of the symbol names. If the value is 
nonzero, the value represents a string table index that gives the symbol 
name. Otherwise, the symbol table entry has no name.

So, the function name (including @..., @@...) should be in string table
which is the same for the above two pthread_cond_signal symbols.

I think it is worthwhile to debug why in your situation 
pthread_create@GLIBC_2.2.5 and pthread_create@@GLIBC_2.34 do not
have them in the string table.

> 
> This patch reworks how we compare the symbol name provided by the user if it is
> qualified with a version (using @ or @@). We now look up the correct version
> string in the version symbol table before constructing the full name, as also
> done above by nm, before comparing.
> 
>>> version information would be present in the string found in the
>>> string table.
>>>
>>> We now look up the correct version string in the version symbol
>>> table before constructing the full name and then comparing.
>>>
>>> This patch adds support for both name@version and name@@version to
>>> match output of the various elf parsers.
>>>
>>> Signed-off-by: Espen Grindhaug <espen.grindhaug@gmail.com>
>>
>> [...]