Re: Unsafe access BufferDescriptors array in BufferGetLSNAtomic()

On Thu, Nov 7, 2024 at 7:07 PM Tender Wang <tndrwang(at)gmail(dot)com> wrote:
> While learning gist index insert codes, I find a little issue with BufferGetLSNAtomic().
> At first, it wants to get bufHdr by accessing the buffer descriptor array, as below:
>
> BufferDesc *bufHdr = GetBufferDescriptor(buffer - 1);
>
> However, it doesn't check whether the passed buffer is a local or shared buffer.
> If the buffer is local, then buffer < 0; it will be cast to uint32 when
> passed to GetBufferDescriptor().
> This may be unsafe, although no someone reports the problem.
>
> I tweak a few codes; see the attached patch.
> Any thoughts?

LGTM. When considering a local buffer, the GetBufferDescriptor() call
in BufferGetLSNAtomic() would be retrieving a shared buffer with a bad
buffer ID. Since the code checks whether the buffer is shared before
using the retrieved BufferDesc, this issue did not lead to any
malfunction. Nonetheless this seems like trouble waiting to happen.

Thanks
Richard