[Bug tools/28873] New: Implement eu-readelf -D

[Bug tools/28873] New: Implement eu-readelf -D

[dichen at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

            Bug ID: 28873
           Summary: Implement eu-readelf -D
           Product: elfutils
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: tools
          Assignee: unassigned at sourceware dot org
          Reporter: dichen at redhat dot com
                CC: elfutils-devel at sourceware dot org
  Target Milestone: ---

Currently, eu-readelf is using section headers (in readelf.c:handle_symtab(),
it calls gelf_getshdr() for fetching section headers and get related info like
section size, type, etc) to find the symbol information and print them.

This task will add new options to eu-readelf (-D and --use-dynamic).

And this task aims for printing the symbols by lookup through the program
headers
and the dynamic table. PT_DYNAMIC->DT_* for DT_{GNU_,}HASH, DT_SYMTAB,
DT_STRSZ,
DT_VERNEED, DT_VERSYM(for version information), etc.

previous discussion: https://bugzilla.redhat.com/show_bug.cgi?id=444621

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[Bug tools/28873] Implement eu-readelf -D

[dichen at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

Di Chen <dichen at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED

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[Bug tools/28873] Implement eu-readelf -D

[dichen at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

Di Chen <dichen at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at sourceware dot org   |dichen at redhat dot com

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[Bug tools/28873] Implement eu-readelf -D

[dichen at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

Di Chen <dichen at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Depends on|                            |28928


Referenced Bugs:

https://sourceware.org/bugzilla/show_bug.cgi?id=28928
[Bug 28928] Wrong dynamic section entry number
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[Bug tools/28873] Implement eu-readelf -D

[dichen at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

--- Comment #1 from Di Chen <dichen at redhat dot com> ---
This Bug depends on Bug 28928 because when calling 
`$ eu-readelf -d --use-dynamic {FILE}`
eu-readelf will need the number of dynamic section entris which involves with
Bug 28928

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[Bug tools/28873] Implement eu-readelf -D

[mark at klomp dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873
Bug 28873 depends on bug 28928, which changed state.

Bug 28928 Summary: Wrong dynamic section entry number
https://sourceware.org/bugzilla/show_bug.cgi?id=28928

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

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[Bug tools/28873] Implement eu-readelf -D

[dichen at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

--- Comment #2 from Di Chen <dichen at redhat dot com> ---
commit 369c021c6eedae3665c1dbbaa4fc43afbbb698f4
Author: Di Chen <dichen@redhat.com>
Date:   Thu Apr 28 19:55:33 2022 +0800

    readelf: Support --dynamic with --use-dynamic

    Currently, eu-readelf is using section headers to dump the dynamic
    segment information (print_dynamic -> handle_dynamic).

    This patch adds new options to eu-readelf (-D, --use-dynamic)
    for (-d, --dynamic).

    https://sourceware.org/bugzilla/show_bug.cgi?id=28873

    Signed-off-by: Di Chen <dichen@redhat.com>

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[Bug tools/28873] Implement eu-readelf -D

[dichen at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

--- Comment #3 from Di Chen <dichen at redhat dot com> ---
Finished the first part of the whole task, now eu-readelf can dump the dynamic
segment information from dynamic program header, i.e.

```
$ ./src/readelf -Dd ~/test/eu-readelf-no-shdr 

Dynamic segment contains 26 entries:
 Addr: 0x0000000000474e00  Offset: 0x073e00
  Type              Value
  NEEDED            Shared library: [libdw.so.1]
  NEEDED            Shared library: [libelf.so.1]
  NEEDED            Shared library: [libc.so.6]
  INIT              0x0000000000404000
  FINI              0x000000000043d4a8
  INIT_ARRAY        0x0000000000474df0
  INIT_ARRAYSZ      8 (bytes)
  FINI_ARRAY        0x0000000000474df8
  FINI_ARRAYSZ      8 (bytes)
  GNU_HASH          0x00000000004003a0
  STRTAB            0x00000000004016b0
  SYMTAB            0x00000000004003f0
  STRSZ             3086 (bytes)
  SYMENT            24 (bytes)
  DEBUG             
  PLTGOT            0x0000000000475000
  PLTRELSZ          4560 (bytes)
  PLTREL            RELA
  JMPREL            0x00000000004026a0
  RELA              0x0000000000402610
  RELASZ            144 (bytes)
  RELAENT           24 (bytes)
  VERNEED           0x0000000000402450
  VERNEEDNUM        3
  VERSYM            0x00000000004022be
  NULL              

```

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[Bug tools/28873] Implement eu-readelf -D

[dichen at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

--- Comment #4 from Di Chen <dichen at redhat dot com> ---
Update:

I am working on the second part of the task, make eu-readelf can dump symbol
information from dynamic program header, like $ eu-readelf -Ds {FILE}

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[Bug tools/28873] Implement eu-readelf -D

[dichen at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

--- Comment #5 from Di Chen <dichen at redhat dot com> ---
Created attachment 14600
  --> https://sourceware.org/bugzilla/attachment.cgi?id=14600&action=edit
readelf: display dynamic symtab without section headers

```
# failed to print symtab of a binary which has section headers removed.
$ ./src/readelf -s ~/test/a.out

# it works with "-D" for a binary which has section headers removed.
$ ./src/readelf -Ds ~/test/a.out 
    0: 0000000000000000      0 NOTYPE  LOCAL  DEFAULT    UNDEF 
    1: 0000000000000000      0 FUNC    GLOBAL DEFAULT    UNDEF
__libc_start_main@GLIBC_2.34 (2)
    2: 0000000000000000      0 NOTYPE  WEAK   DEFAULT    UNDEF __gmon_start__
```

It works well for binaries which has SYMTAB section right after STRTAB section.
```
$ readelf -Dd ~/test/a.out 

Dynamic section at offset 0x2e60 contains 20 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 0x000000000000000c (INIT)               0x401000
 0x000000000000000d (FINI)               0x401124
 0x0000000000000019 (INIT_ARRAY)         0x403e50
 0x000000000000001b (INIT_ARRAYSZ)       8 (bytes)
 0x000000000000001a (FINI_ARRAY)         0x403e58
 0x000000000000001c (FINI_ARRAYSZ)       8 (bytes)
 0x000000006ffffef5 (GNU_HASH)           0x4003c0
 0x0000000000000005 (STRTAB)             0x400428
 0x0000000000000006 (SYMTAB)             0x4003e0
 0x000000000000000a (STRSZ)              55 (bytes)
 0x000000000000000b (SYMENT)             24 (bytes)
 0x0000000000000015 (DEBUG)              0x0
 0x0000000000000007 (RELA)               0x400488
 0x0000000000000008 (RELASZ)             48 (bytes)
 0x0000000000000009 (RELAENT)            24 (bytes)
 0x000000006ffffffe (VERNEED)            0x400468
 0x000000006fffffff (VERNEEDNUM)         1
 0x000000006ffffff0 (VERSYM)             0x400460
 0x0000000000000000 (NULL)               0x0
```
Because I use offset difference between SYMTAB and STRTAB to get the symbol
table entry number.
```
  size_t syments = ((offs[i_strtab] - offs[i_symtab]) /
    gelf_fsize(ebl->elf, ELF_T_SYM, 1, EV_CURRENT));
```

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[Bug tools/28873] Implement eu-readelf -D

[dichen at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

--- Comment #6 from Di Chen <dichen at redhat dot com> ---
[Follow the last comment]

For a binary with SYMTAB STRTAB having different order, like upside down, or
having other section in between. eg.

```
$ readelf -Dd /usr/local/go/bin/go

Dynamic section at offset 0x9e5220 contains 19 entries:
  Tag        Type                         Name/Value
 0x0000000000000004 (HASH)               0xb3c3c0
 0x0000000000000006 (SYMTAB)             0xb3c880
 0x000000000000000b (SYMENT)             24 (bytes)
 0x0000000000000005 (STRTAB)             0xb3c660
 0x000000000000000a (STRSZ)              531 (bytes)
 0x0000000000000007 (RELA)               0xb3bfd8
 0x0000000000000008 (RELASZ)             24 (bytes)
 0x0000000000000009 (RELAENT)            24 (bytes)
 0x0000000000000003 (PLTGOT)             0xde5100
 0x0000000000000015 (DEBUG)              0x0
 0x0000000000000001 (NEEDED)             Shared library: [libpthread.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 0x000000006ffffffe (VERNEED)            0xb3c360
 0x000000006fffffff (VERNEEDNUM)         2
 0x000000006ffffff0 (VERSYM)             0xb3c300
 0x0000000000000014 (PLTREL)             RELA
 0x0000000000000002 (PLTRELSZ)           768 (bytes)
 0x0000000000000017 (JMPREL)             0xb3bff0
 0x0000000000000000 (NULL)               0x0

```

It will mess up the syments (symbol table entry number) calculation.

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[Bug tools/28873] Implement eu-readelf -D

[amerey at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

Aaron Merey <amerey at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amerey at redhat dot com

--- Comment #7 from Aaron Merey <amerey at redhat dot com> ---
(In reply to Di Chen from comment #5)
> Because I use offset difference between SYMTAB and STRTAB to get the symbol
> table entry number.
> ```
>   size_t syments = ((offs[i_strtab] - offs[i_symtab]) /
>     gelf_fsize(ebl->elf, ELF_T_SYM, 1, EV_CURRENT));
> ```
>
> For a binary with SYMTAB STRTAB having different order, like upside down, or
> having other section in between. eg.
> [...]
> It will mess up the syments (symbol table entry number) calculation.

This raises an interesting question: how do you calculate the number of symbols
in .dynsym without using section headers?

I figured there'd some kind of "DT_SYMTABNUM" value somewhere but unfortunately
the answer doesn't appear to be so straightforward.

Judging from the binutils readelf source code you need to use information in
the .hash and .gnu.hash sections to calculate the number of entries. 

To complicate things even more, a binary can contain either .hash or .gnu.hash
or both and computing the number of .dynsym entries is different in each case.
See binutils/readelf.c:get_num_dynamic_syms, you may need to implement some of
this in your patch.

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[Bug tools/28873] Implement eu-readelf -D

[mark at klomp dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

--- Comment #8 from Mark Wielaard <mark at klomp dot org> ---
(In reply to Aaron Merey from comment #7) 
> This raises an interesting question: how do you calculate the number of
> symbols in .dynsym without using section headers?
> 
> I figured there'd some kind of "DT_SYMTABNUM" value somewhere but
> unfortunately the answer doesn't appear to be so straightforward.

It has been proposed, but not (yet) adopted:
https://groups.google.com/g/generic-abi/c/9L03yrxXPBc
(sorry, a google groups link, there should be a normal archive, but I cannot
find it right now). If that was adopted and linkers would generate it, then
this question would indeed have a simple answer. Sadly, it isn't :{

> Judging from the binutils readelf source code you need to use information in
> the .hash and .gnu.hash sections to calculate the number of entries. 
> 
> To complicate things even more, a binary can contain either .hash or
> .gnu.hash or both and computing the number of .dynsym entries is different
> in each case. See binutils/readelf.c:get_num_dynamic_syms, you may need to
> implement some of this in your patch.

If there is a .hash section then it is fairly easy, the first word is the
number of symbols the hash/symbol table describes.

If it is a .gnu.hash section then sadly you have to parse and go through the
whole hashtable and count.

There is an implementation already in elfutils, but it is a bit hiden and
obscure if you don't know what you are looking for. Search for "Figure out the
size of the symbol table" in libdwfl/dwfl_module_getdwarf.c.

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[Bug tools/28873] Implement eu-readelf -D

[mark at klomp dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28873

Mark Wielaard <mark at klomp dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #9 from Mark Wielaard <mark at klomp dot org> ---
commit 4d8de4b2fa05495d69d09e1a3d335f24d6bf33ee
Author: Di Chen <dichen@redhat.com>
Date:   Mon Mar 27 10:01:05 2023 +0800

    readelf: display dynamic symtab without section headers

    This commit adds a new option "-D/--use-dynamic" to support printing the
    dynamic symbol table from the PT_DYNAMIC segment. By using the
    PT_DYNAMIC segment, eu-readelf can go through the contents of dynamic
    section entries and the values of each tag. From that, we can get the
    address and size of the dynamic symbol table, the address of the string
    table, etc.

    By using the new option "-D/--use-dynamic", eu-readelf can list the
    symbols without section headers.

    Example:
      $ ./src/readelf -Ds a.out
          0: 0000000000000000      0 NOTYPE  LOCAL  DEFAULT    UNDEF
          1: 0000000000000000      0 FUNC    GLOBAL DEFAULT    UNDEF
__libc_start_main@GLIBC_2.34 (2)
          2: 0000000000000000      0 NOTYPE  WEAK   DEFAULT    UNDEF
__gmon_start__

    https://sourceware.org/bugzilla/show_bug.cgi?id=28873

    Signed-off-by: Di Chen <dichen@redhat.com>

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