[PING][PATCH] [gdb/tdep] Fix inferior plt calls in PIE for i386

[Tom de Vries <tdevries@suse.de>]

On 11/1/21 1:00 PM, Tom de Vries via Gdb-patches wrote:
> Consider test-case test.c:
> ...
> int main (void) {
>   void *p = malloc (10);
>   return 0;
> }
> ...
> 
> When compiled to a non-PIE exec:
> ...
> $ gcc -m32 test.c
> ...
> the call sequence looks like:
> ...
>  8048447:       83 ec 0c                sub    $0xc,%esp
>  804844a:       6a 0a                   push   $0xa
>  804844c:       e8 bf fe ff ff          call   8048310 <malloc@plt>
> ...
> which calls to:
> ...
> 08048310 <malloc@plt>:
>  8048310:       ff 25 0c a0 04 08       jmp    *0x804a00c
>  8048316:       68 00 00 00 00          push   $0x0
>  804831b:       e9 e0 ff ff ff          jmp    8048300 <.plt>
> ...
> where the first insn at 0x8048310 initially jumps to the following address
> 0x8048316, read from the .got.plt @ 0x804a00c:
> ...
>  804a000 0c9f0408 00000000 00000000 16830408  ................
>  804a010 26830408                             &...
> ...
> 
> Likewise, when compiled as a PIE:
> ...
> $ gcc -m32 -fPIE -pie test.c
> ...
> we have this call sequence (with %ebx setup to point to the .got.plt):
> ...
> 0000055d <main>:
>  579:   83 ec 0c                sub    $0xc,%esp
>  57c:   6a 0a                   push   $0xa
>  57e:   89 c3                   mov    %eax,%ebx
>  580:   e8 6b fe ff ff          call   3f0 <malloc@plt>
> ...
> which calls to:
> ...
> 000003f0 <malloc@plt>:
>  3f0:   ff a3 0c 00 00 00       jmp    *0xc(%ebx)
>  3f6:   68 00 00 00 00          push   $0x0
>  3fb:   e9 e0 ff ff ff          jmp    3e0 <.plt>
> ...
> where the insn at 0x3f0 initially jumps to following address 0x3f6, read from
> the .got.plt at offset 0xc:
> ...
>  2000 f41e0000 00000000 00000000 f6030000  ................
>  2010 06040000                             ....
> ...
> 
> When instead doing an inferior call to malloc (with nosharedlib to force
> malloc to resolve to malloc@plt rather than the functions in ld.so or libc.so)
> with the non-PIE exec, we have the expected:
> ...
> $ gdb -q -batch a.out -ex start -ex nosharedlib -ex "p /x (void *)malloc (10)"
> Temporary breakpoint 1 at 0x8048444
> 
> Temporary breakpoint 1, 0x08048444 in main ()
> $1 = 0x804b160
> ...
> 
> But with the PIE exec, we run into:
> ...
> $ gdb -q -batch a.out -ex start -ex nosharedlib -ex "p /x (void *)malloc (10)"
> Temporary breakpoint 1 at 0x56c
> 
> Temporary breakpoint 1, 0x5655556c in main ()
> 
> Program received signal SIGSEGV, Segmentation fault.
> 0x565553f0 in malloc@plt ()
> ...
> 
> The segfault happens because:
> - the inferior call mechanism doesn't setup %ebx
> - %ebx instead is 0
> - the jump to "*0xc(%ebx)" reads from memory at 0xc
> 
> Fix this by setting up %ebx properly in i386_thiscall_push_dummy_call.
> 
> Fixes this failure with target board unix/-m32/-pie/-fPIE reported in
> PR28467:
> ...
> FAIL: gdb.base/nodebug.exp: p/c (int) array_index("abcdef",2)
> ...
> 
> Tested on x86_64-linux, with target board unix/-m32 and unix/-m32/-fPIE/-pie.
> 

Ping.  Any comment?

Thanks,
- Tom

> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28467
> ---
>  gdb/i386-tdep.c | 43 +++++++++++++++++++++++++++++++++++++++++++
>  gdb/maint.c     |  2 +-
>  gdb/maint.h     |  3 +++
>  3 files changed, 47 insertions(+), 1 deletion(-)
> 
> diff --git a/gdb/i386-tdep.c b/gdb/i386-tdep.c
> index f65a07492d2..7df9420eba2 100644
> --- a/gdb/i386-tdep.c
> +++ b/gdb/i386-tdep.c
> @@ -67,6 +67,8 @@
>  #include <algorithm>
>  #include <unordered_set>
>  #include "producer.h"
> +#include "infcall.h"
> +#include "maint.h"
>  
>  /* Register names.  */
>  
> @@ -2776,6 +2778,47 @@ i386_thiscall_push_dummy_call (struct gdbarch *gdbarch, struct value *function,
>      regcache->cooked_write (I386_ECX_REGNUM,
>  			    value_contents_all (args[0]).data ());
>  
> +  /* If the PLT is position-independent, the SYSTEM V ABI requires %ebx to be
> +     set to the address of the GOT when doing a call to a PLT address.
> +     Note that we do not try to determine whether the PLT is
> +     position-independent, we just set the register regardless.  */
> +  CORE_ADDR func_addr = find_function_addr (function, nullptr, nullptr);
> +  if (in_plt_section (func_addr))
> +    {
> +      struct objfile *objf = nullptr;
> +      asection *asect = nullptr;
> +      obj_section *osect = nullptr;
> +
> +      /* Get object file containing func_addr.  */
> +      obj_section *func_section = find_pc_section (func_addr);
> +      if (func_section != nullptr)
> +	objf = func_section->objfile;
> +
> +      if (objf != nullptr)
> +	{
> +	  /* Get corresponding .got.plt or .got section.  */
> +	  asect = bfd_get_section_by_name (objf->obfd, ".got.plt");
> +	  if (asect == nullptr)
> +	    asect = bfd_get_section_by_name (objf->obfd, ".got");
> +	}
> +
> +      if (asect != nullptr)
> +	/* Translate asection to obj_section.  */
> +	osect = maint_obj_section_from_bfd_section (objf->obfd, asect, objf);
> +
> +      if (osect != nullptr)
> +	{
> +	  /* Store the section address in %ebx.  */
> +	  store_unsigned_integer (buf, 4, byte_order, osect->addr ());
> +	  regcache->cooked_write (I386_EBX_REGNUM, buf);
> +	}
> +      else
> +	{
> +	  /* If we would only do this for a position-independent PLT, it would
> +	     make sense to issue a warning here.  */
> +	}
> +    }
> +
>    /* MarkK wrote: This "+ 8" is all over the place:
>       (i386_frame_this_id, i386_sigtramp_frame_this_id,
>       i386_dummy_id).  It's there, since all frame unwinders for
> diff --git a/gdb/maint.c b/gdb/maint.c
> index bcc71aab579..75d3e49991b 100644
> --- a/gdb/maint.c
> +++ b/gdb/maint.c
> @@ -329,7 +329,7 @@ print_objfile_section_info (bfd *abfd, struct obj_section *asect,
>     from ABFD.  It might be that no such wrapper exists (for example debug
>     sections don't have such wrappers) in which case nullptr is returned.  */
>  
> -static obj_section *
> +obj_section *
>  maint_obj_section_from_bfd_section (bfd *abfd,
>  				    asection *asection,
>  				    objfile *ofile)
> diff --git a/gdb/maint.h b/gdb/maint.h
> index d3c0122a321..81b3beb703d 100644
> --- a/gdb/maint.h
> +++ b/gdb/maint.h
> @@ -63,4 +63,7 @@ class scoped_command_stats
>    int m_start_nr_blocks;
>  };
>  
> +extern obj_section *maint_obj_section_from_bfd_section (bfd *abfd,
> +							asection *asection,
> +							objfile *ofile);
>  #endif /* MAINT_H */
> 
> base-commit: 94c9216c03ab1af16b1bdd11a10a66c13e6458d8
>