[lldb-dev] File::Read does not read everything

[Eugene Birukov via lldb-dev]

Here is what I am running now:


diff --git a/source/Host/common/File.cpp b/source/Host/common/File.cpp
index 89587a9..a4ac25e 100644
--- a/source/Host/common/File.cpp
+++ b/source/Host/common/File.cpp
@@ -771,21 +771,34 @@ File::Read (void *buf, size_t &num_bytes, off_t &offset)
     int fd = GetDescriptor();
     if (fd != kInvalidDescriptor)
     {
-        ssize_t bytes_read = -1;
-        do
+        size_t bytes_left = num_bytes;
+        num_bytes = 0;
+        char *pos = (char*)buf;
+        while (bytes_left > 0)
         {
-            bytes_read = ::pread (fd, buf, num_bytes, offset);
-        } while (bytes_read < 0 && errno == EINTR);
+            ssize_t bytes_read = -1;
+            do
+            {
+                bytes_read = ::pread (fd, pos, bytes_left, offset);
+            } while (bytes_read < 0 && errno == EINTR);

-        if (bytes_read < 0)
-        {
-            num_bytes = 0;
-            error.SetErrorToErrno();
-        }
-        else
-        {
-            offset += bytes_read;
-            num_bytes = bytes_read;
+            if (bytes_read < 0)
+            {
+                num_bytes = 0;
+                error.SetErrorToErrno();
+                break;
+            }
+            else if (bytes_read == 0)
+            {
+                break;
+            }
+            else
+            {
+                offset += bytes_read;
+                num_bytes += bytes_read;
+                bytes_left -= bytes_read;
+                pos += bytes_read;
+            }
         }
     }
     else




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________________________________
From: Greg Clayton <gclayton at apple.com>
Sent: Monday, October 17, 2016 12:39 PM
To: Eugene Birukov
Cc: LLDB
Subject: Re: [lldb-dev] File::Read does not read everything

It is probably best to make the code loop as long as "bytes_reads > 0" and we haven't read "num_bytes" yet. The darwin kernel has a INT32_MAX read size which gets set to MAX_READ_SIZE, but we need to do that because if you try to read more than that it reads nothing. So MAX_READ_SIZE is more for the case where the read will outright fail if given a byte size that is too large. Feel free to submit a patch that can keep calling pread correctly in a loop as long as bytes_read > 0.

Greg

> On Oct 17, 2016, at 12:20 PM, Eugene Birukov via lldb-dev <lldb-dev at lists.llvm.org> wrote:
>
> Hello,
>
> I am using LLDB 3.9 on Linux Ubuntu. I am loading a 5GiB core which is located on Windows file share mounted on Linux via mount.cifs. I see that we successfully allocated memory and are trying to fill it in one read. Unfortunately pread returns 2GiB and we never check for short read here.
>
> I think we could combine this code with previous block "#if defined (MAX_READ_SIZE)" if we simply check for short read here (bytes_read too small) and loop until all the buffer is filled.
>
> Thanks,
> Eugene
>
> #ifndef _WIN32
>     int fd = GetDescriptor();
>     if (fd != kInvalidDescriptor)
>     {
>         ssize_t bytes_read = -1;
>         do
>         {
>             bytes_read = ::pread (fd, buf, num_bytes, offset);
>         } while (bytes_read < 0 && errno == EINTR);
>
>         if (bytes_read < 0)
>         {
>             num_bytes = 0;
>             error.SetErrorToErrno();
>         }
>         else
>         {
>             offset += bytes_read;
>             num_bytes = bytes_read;
>         }
>     }
>
> Sent from Outlook
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