[PATCH] linux_nat_target::xfer_partial: Fallback to ptrace

[Keith Seitz]

Ping

On 6/3/22 08:18, Keith Seitz via Gdb-patches wrote:
> Pedro Alves wrote:
> 
>> I guess the write to /proc/pid/mem fails with EIO for you, and there's nothing
>> else we can use to detect the scenario.  So we probably want to check TARGET_XFER_E_IO
>> instead.  And, maybe only do the fallback if writing.
> 
> I've updated the patch to include these changes and retested on all the "usual"
> target boards on Fedora 36 x86_64, s390x, ppc64le, and aarch64 and on RHEL6
> x86_64.
> 
> Keith
> 
> --
> 
> Commit 05c06f318fd9a112529dfc313e6512b399a645e4 enabled GDB
> to access memory while threads are running. It did this by accessing
> /proc/PID/task/LWP/mem.
> 
> Unfortunately, this interface is not implemented for writing in older kernels
> (such as RHEL6). This means that GDB is unable to insert breakpoints on
> these hosts:
> 
> $ ./gdb -q gdb -ex start
> Reading symbols from gdb...
> Temporary breakpoint 1 at 0x40fdd5: file ../../src/gdb/gdb.c, line 28.
> Starting program: /home/rhel6/fsf/linux/gdb/gdb
> Warning:
> Cannot insert breakpoint 1.
> Cannot access memory at address 0x40fdd5
> 
> (gdb)
> 
> Before this patch, linux_proc_xfer_memory_partial (previously called
> linux_proc_xfer_partial) would return TARGET_XFER_EOF if the write
> to /proc/PID/mem failed. [More specifically, linux_proc_xfer_partial would
> not "bother for one word," but the effect is the essentially same.]
> 
> This status was checked by linux_nat_target::xfer_partial, which would then
> fallback to using ptrace to perform the operation.
> 
> This is the specific hunk that removed the fallback:
> 
> -  xfer = linux_proc_xfer_partial (object, annex, readbuf, writebuf,
> -                                 offset, len, xfered_len);
> -  if (xfer != TARGET_XFER_EOF)
> -    return xfer;
> +      return linux_proc_xfer_memory_partial (readbuf, writebuf,
> +                                            offset, len, xfered_len);
> +    }
> 
>     return inf_ptrace_target::xfer_partial (object, annex, readbuf, writebuf,
>                                            offset, len, xfered_len);
> 
> This patch restores this fallback mechanism, enabling GDB to insert
> breakpoints on these older kernels. Note that a recent patch changed
> the return status from TARGET_XFER_EOF to TARGET_XFER_E_IO.
> 
> Tested on {unix,native-gdbserver,native-extended-gdbserver}/-m{32,64}
> on x86_64, s390x, aarch64, and ppc64le.
> ---
>   gdb/linux-nat.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
> index 3b5400896bc..571c97137c2 100644
> --- a/gdb/linux-nat.c
> +++ b/gdb/linux-nat.c
> @@ -3706,8 +3706,12 @@ linux_nat_target::xfer_partial (enum target_object object,
>         if (addr_bit < (sizeof (ULONGEST) * HOST_CHAR_BIT))
>   	offset &= ((ULONGEST) 1 << addr_bit) - 1;
>   
> -      return linux_proc_xfer_memory_partial (readbuf, writebuf,
> -					     offset, len, xfered_len);
> +      enum target_xfer_status xfer
> +	= linux_proc_xfer_memory_partial (readbuf, writebuf,
> +					  offset, len, xfered_len);
> +      if (xfer != TARGET_XFER_E_IO || readbuf != nullptr)
> +	return xfer;
> +      /* Fallthrough to ptrace.  /proc/pid/mem wasn't writable before Linux 2.6.39.  */
>       }
>   
>     return inf_ptrace_target::xfer_partial (object, annex, readbuf, writebuf,