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

[Yonghong Song]

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@xxxxxxxxx>
> >
> > [...]