<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/126629>126629</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            `#embed` doesn't support reading from devices
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          zygoloid
      </td>
    </tr>
</table>

<pre>
    Clang's `#embed` implementation doesn't behave properly when applied to a device:
```c++
int per_build_entropy[] = {
#embed "/dev/random" limit(1024)
};
```
Instead it treats devices as being empty -- in the above case, this leads to the array being initialized with zeroes! This is probably because we're using the `st_size` field produced by `stat`, but that only lists the size of the file if applied to a regular file:
> `st_size`
> > This field gives the size of the file (if it is a regular file or a symbolic link) in bytes.  The size of a symbolic link is the length of the pathname it contains, without a terminating null byte.

While we try to [detect this case](https://github.com/llvm/llvm-project/blob/3c6d1dd362009e0aebd91c5197d40f8ce59fcff4/llvm/lib/Support/MemoryBuffer.cpp#L493) and use stream IO instead of mmap, we only do so when we weren't given a `MapSize`, which in this case we presumably will be due to the `limit(1024)`. And the code that handles the case where we have no `MapSize` does *not* handle the case where we're opening a device rather than a regular file. Perhaps `shouldUseMmap` should return `false` unless it's operating on a regular file?
</pre>
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