Re: [edk2-devel] [PATCH v1 1/1] ArmPkg: DefaultExceptionHandlerLib: Do Not Allocate\r Memory

["Ard Biesheuvel" <ardb@kernel.org>]

On Wed, 2 Aug 2023 at 18:21, Oliver Smith-Denny
<osde@linux.microsoft.com> wrote:
>
> On 8/2/2023 9:06 AM, Ard Biesheuvel wrote:
> > On Tue, 1 Aug 2023 at 00:04, Oliver Smith-Denny
> > <osde@linux.microsoft.com> wrote:
> >>
> >> If gST->ConOut is available when Arm's DefaultExceptionHandler
> >> is running, AsciiPrint will get called to attempt to print to
> >> ConOut, in addition to the serial output.
> >>
> >> AsciiPrint calls AsciiInternalPrint in UefiLibPrint.c which
> >> in turn calls AllocatePool to allocate a buffer to convert
> >> the Ascii input string to a Unicode string to pass to
> >> ConOut->OutputString.
> >>
> >> Per the comment on DefaultExceptionHandler, we should not be
> >> allocating memory in the exception handler, as this can cause
> >> the exception handler to fail if we had a memory exception or
> >> the system state is such that we cannot allocate memory.
> >>
> >> It has been observed on ArmVirtQemu that exceptions generated
> >> in the memory handling code will fail to output the stack dump
> >> and CPU state that is critical to debugging because the
> >> AllocatePool will fail.
> >>
> >> This patch fixes the Arm and AARCH64 DefaultExceptionHandlers to
> >> not allocate memory when ConOut is available and instead use
> >> stack memory to convert the Ascii string needed for SerialPortWrite
> >> to the Unicode string needed for ConOut->OutputString. Correspondingly,
> >> ArmVirtQemu can now output the stack dump and CPU state when hitting
> >> an exception in memory code.
> >>
> >> GitHub PR: https://github.com/tianocore/edk2/pull/4703
> >>
> >> Cc: Leif Lindholm <quic_llindhol@quicinc.com>
> >> Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
> >> Cc: Sami Mujawar <sami.mujawar@arm.com>
> >> Cc: Gerd Hoffmann <kraxel@redhat.com>
> >>
> >> Signed-off-by: Oliver Smith-Denny <osde@linux.microsoft.com>
> >
> > Thanks a lot for fixing this.
> >
> > Is calling into gST->ConOut guaranteed to be safe in this regard? Or
> > is it still best effort?
> >
> >
>
> Yeah, this is something I worried about when fixing this. It is very
> much best effort, because there are no guarantees that OutputString will
> not allocate memory (or some other such unsafe operation in an exception
> handler). With the ability to BYO ConOut stack and the complications
> with graphics that can be involved here, I personally would be happy to
> drop the ConOut call entirely and rely on the serial output.
>
> In my testing, this did work on ArmVirtQemu and I can envision a case
> where having the ConOut output would be nice, but I tend towards lets
> keep the exception handler as simple as possible and make sure we get
> the useful information dumped out.
>
> Another option would be to keep this change but move the ConOut calls
> to after all of the serial output, so that if we do get a recursive
> exception in ConOut, at least we got our serial output.
>
> Let me know what you think, happy to spin up a v2.
>

Yeah, that seems like a nice approach - it will still be best effort,
but it won't derail the more reliable serial console if it fails.

That probably also means we shouldn't bother printing the 'recursive
exception' to ConOut at all, or perhap have a special 'recursive
exception in conout' flag on top of the existing one but that might be
overkill.


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