[lldb-dev] PATCH for REVIEW - Fix [Bug 15038] LLDB does not support printing wide-character variables on Linux

Enrico Granata egranata at apple.com
Thu Mar 28 11:38:45 PDT 2013

now I pretty much see where you are coming from.
What we have here is a Linux-specific implementation issue (which of course is not going to get fixed anytime soon :-)

What troubles me a little with the overall idea of this patch is that we are putting a workaround for a lower-level issue in higher-level code.
If we go this route, the same thing might have to be done for ValueObject::ReadPointedString() which does the same kind of “unbounded read”
I would prefer, if we do this, that at least we put the code around #if defined (__linux__), as in:
     if (error.Fail() || data_read == 0)
+        bool found = false;
#if defined (__linux__)
+        if (data_read && error.GetType() == eErrorTypePOSIX) {
+            // Determine if a null terminator was found in spite of an out-of-bounds read
+            if (error.GetError() == EIO || error.GetError() == EFAULT) {
+                char* buffer = reinterpret_cast<char *>(buffer_sp->GetBytes());
+                char terminator[4] = {'\0', '\0', '\0', '\0'};
+                assert(sizeof(terminator) >= type_size && "Atempting to read a wchar with more than 4 bytes per character!");
+                for (size_t i = 0; !found && (i + type_size <= data_read); i += type_size)
+                    if (::strncmp(&buffer[i], terminator, type_size) == 0)
+                        found = true; // null terminator
+            }
+        }
+        if (!found) {
+            stream.Printf("unable to read data");
+            return true;

This will maintain a clean behavior for non-Linux systems (and other systems with a similarly broken API can opt-in this behavior by adding themselves to the ifdef)

But I am inclined to believe the best option would be to adopt your ProcessMonitor fix that reads smaller and smaller chunks and then returns the number of bytes read and no error - that would make all unbounded-read-based calls just work without having to add Linux-specific patches around (main issue being that now we all have to remember that anything that reads a string-like thing has to add a similar workaround or cause Linux unhappiness)

If that is not going to be viable, or will take some time, I don’t want to keep you blocked, so I would suggest:
- commit this patch with #ifdef markers to keep it Linux-only
- file a bugzilla issue to make process reads as functional as possible on Linux


Enrico Granata
✉ egranata@.com
✆ 27683

On Mar 28, 2013, at 8:39 AM, "Thirumurthi, Ashok" <ashok.thirumurthi at intel.com> wrote:

> Thanks for the feedback, Enrico,
> I’m relying on the fact that when the wstring is larger than the default size (1GB in your example), we won’t get a ptrace error like EIO or EFAULT.  So, there will be no search for a null terminator.
> If we can’t rely on the null terminator, it’s hard to know if a different error occurred.  The trouble is that there is inconsistency in the PTRACE_PEEK implementation on Linux that is more or less arbitrary (sigh), so the out-of-bounds read can generate EIO or EFAULT.  In particular, EIO can mean a number of things:
>   “request is invalid, or an attempt was made to read from or write to an invalid area in the parent's or child's memory, or there was a word-alignment violation, or an invalid signal was specified during a restart request.” –http://linux.die.net/man/2/ptrace            
> In addition, the Linux ProcessMonitor reads in 8-byte chunks, so ReadUTFBufferAndDumpToStream will probably fail (for the right reasons) in the case where at least part of the null terminator was in that block.  I see that as an implementation detail that can be resolved in ProcessMonitor by reading smaller chunks until PTRACE_PEEK succeeds (i.e. a good follow-up patch).
> Do let me know if you feel this is okay to commit as is.  Cheers,
> -        Ashok
> From: Enrico Granata [mailto:egranata at apple.com] 
> Sent: Wednesday, March 27, 2013 6:18 PM
> To: Thirumurthi, Ashok
> Cc: lldb-dev at cs.uiuc.edu
> Subject: Re: [lldb-dev] PATCH for REVIEW - Fix [Bug 15038] LLDB does not support printing wide-character variables on Linux
> Hi Ashok,
> thanks for submitting an LLDB patch :)
> I have looked at the code and while I see where the patch is coming from, I see a potential issue with it.
> The code in ReadUTFBufferAndDumpToStream, which is what you are editing, is meant to work on a partial string (i.e. one that does not have a NULL terminator at the end). The reason for that is that you might have a wstring that is 1GB long and we would not want to try and read all of it and then display it. What we do is pick a size and only extract that much data. For obvious reasons, your string might be longer than our upper boundary, so you would get a chunk of valid bytes and then no end-of-buffer marker.
> It looks like your code would fail in that case and produce no summary for a string if an EIO was received before \0.
> Is there any reason why you can’t just check if (error == EIO and data_read > 0) and if so treat this as a “partial string” condition and keep going?
> Would that break/crash anything?
> Best,
> Enrico Granata
> ✉ egranata@.com
> ✆ 27683
> On Mar 27, 2013, at 2:52 PM, "Thirumurthi, Ashok" <ashok.thirumurthi at intel.com> wrote:
> The root cause for Bug 15038 is a ptrace EIO that can occur because we don't know the size of a (UTF) string and so read a fixed number of characters for strings.  The attached fix accepts EIO in the case where data has been read and a null terminator of the correct size and alignment was found.
> - Ashok
> -----Original Message-----
> From: lldb-dev-bounces at cs.uiuc.edu [mailto:lldb-dev-bounces at cs.uiuc.edu] On Behalf Ofbugzilla-daemon at llvm.org
> Sent: Tuesday, January 22, 2013 10:13 AM
> To: lldb-dev at cs.uiuc.edu
> Subject: [lldb-dev] [Bug 15038] New: LLDB does not support printing wide-character variables on Linux
> http://llvm.org/bugs/show_bug.cgi?id=15038
>             Bug #: 15038
>           Summary: LLDB does not support printing wide-character
>                    variables on Linux
>           Product: lldb
>           Version: unspecified
>          Platform: PC
>        OS/Version: Linux
>            Status: NEW
>          Severity: enhancement
>          Priority: P
>         Component: All Bugs
>        AssignedTo: lldb-dev at cs.uiuc.edu
>        ReportedBy: daniel.malea at intel.com
>    Classification: Unclassified
> Printing a variable of wchar_t type does not behave as expected and results in garbage being printed on the screen.
> To reproduce, remove the @expectedFailureLinux decorator from TestChar1632T.py and TestCxxWCharT.py and run:
> python dotest.py --executable <path-to-lldb> lang/cpp/char1632_t lang/cpp/wchar_t
> --
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