Re: [PATCH v6] Avoid crash calling PrintErrMesg from efi_multiboot2

[Frediano Ziglio <frediano.ziglio@xxxxxxxxx>]

On Thu, Mar 6, 2025 at 2:26 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 26.02.2025 19:54, Marek Marczykowski-Górecki wrote:
> > On Mon, Feb 24, 2025 at 02:31:00PM +0000, Frediano Ziglio wrote:
> >> On Mon, Feb 24, 2025 at 1:16 PM Marek Marczykowski-Górecki
> >> <marmarek@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> >>>
> >>> On Mon, Feb 24, 2025 at 12:57:13PM +0000, Frediano Ziglio wrote:
> >>>> On Fri, Feb 21, 2025 at 8:20 PM Marek Marczykowski-Górecki
> >>>> <marmarek@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> >>>>>
> >>>>> On Mon, Feb 17, 2025 at 04:26:59PM +0000, Frediano Ziglio wrote:
> >>>>>> Although code is compiled with -fpic option data is not position
> >>>>>> independent. This causes data pointer to become invalid if
> >>>>>> code is not relocated properly which is what happens for
> >>>>>> efi_multiboot2 which is called by multiboot entry code.
> >>>>>>
> >>>>>> Code tested adding
> >>>>>>    PrintErrMesg(L"Test message", EFI_BUFFER_TOO_SMALL);
> >>>>>> in efi_multiboot2 before calling efi_arch_edd (this function
> >>>>>> can potentially call PrintErrMesg).
> >>>>>>
> >>>>>> Before the patch (XenServer installation on Qemu, xen replaced
> >>>>>> with vanilla xen.gz):
> >>>>>>   Booting `XenServer (Serial)'Booting `XenServer (Serial)'
> >>>>>>   Test message: !!!! X64 Exception Type - 0E(#PF - Page-Fault)  CPU 
> >>>>>> Apic ID - 00000000 !!!!
> >>>>>>   ExceptionData - 0000000000000000  I:0 R:0 U:0 W:0 P:0 PK:0 SS:0 SGX:0
> >>>>>>   RIP  - 000000007EE21E9A, CS  - 0000000000000038, RFLAGS - 
> >>>>>> 0000000000210246
> >>>>>>   RAX  - 000000007FF0C1B5, RCX - 0000000000000050, RDX - 
> >>>>>> 0000000000000010
> >>>>>>   RBX  - 0000000000000000, RSP - 000000007FF0C180, RBP - 
> >>>>>> 000000007FF0C210
> >>>>>>   RSI  - FFFF82D040467CE8, RDI - 0000000000000000
> >>>>>>   R8   - 000000007FF0C1C8, R9  - 000000007FF0C1C0, R10 - 
> >>>>>> 0000000000000000
> >>>>>>   R11  - 0000000000001020, R12 - FFFF82D040467CE8, R13 - 
> >>>>>> 000000007FF0C1B8
> >>>>>>   R14  - 000000007EA33328, R15 - 000000007EA332D8
> >>>>>>   DS   - 0000000000000030, ES  - 0000000000000030, FS  - 
> >>>>>> 0000000000000030
> >>>>>>   GS   - 0000000000000030, SS  - 0000000000000030
> >>>>>>   CR0  - 0000000080010033, CR2 - FFFF82D040467CE8, CR3 - 
> >>>>>> 000000007FC01000
> >>>>>>   CR4  - 0000000000000668, CR8 - 0000000000000000
> >>>>>>   DR0  - 0000000000000000, DR1 - 0000000000000000, DR2 - 
> >>>>>> 0000000000000000
> >>>>>>   DR3  - 0000000000000000, DR6 - 00000000FFFF0FF0, DR7 - 
> >>>>>> 0000000000000400
> >>>>>>   GDTR - 000000007F9DB000 0000000000000047, LDTR - 0000000000000000
> >>>>>>   IDTR - 000000007F48E018 0000000000000FFF,   TR - 0000000000000000
> >>>>>>   FXSAVE_STATE - 000000007FF0BDE0
> >>>>>>   !!!! Find image based on IP(0x7EE21E9A) (No PDB)  
> >>>>>> (ImageBase=000000007EE20000, EntryPoint=000000007EE23935) !!!!
> >>>>>>
> >>>>>> After the patch:
> >>>>>>   Booting `XenServer (Serial)'Booting `XenServer (Serial)'
> >>>>>>   Test message: Buffer too small
> >>>>>>   BdsDxe: loading Boot0000 "UiApp" from 
> >>>>>> Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(462CAA21-7614-4503-836E-8AB6F4662331)
> >>>>>>   BdsDxe: starting Boot0000 "UiApp" from 
> >>>>>> Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(462CAA21-7614-4503-836E-8AB6F4662331)
> >>>>>>
> >>>>>> This partially rollback commit 00d5d5ce23e6.
> >>>>>>
> >>>>>> Fixes: 9180f5365524 ("x86: add multiboot2 protocol support for EFI 
> >>>>>> platforms")
> >>>>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
> >>>>>
> >>>>> I tried testing this patch, but it seems I cannot reproduce the original
> >>>>> failure...
> >>>>>
> >>>>> I did as the commit message suggests here:
> >>>>> https://gitlab.com/xen-project/people/marmarek/xen/-/commit/ca3d6911c448eb886990f33d4380b5646617a982
> >>>>>
> >>>>> With blexit() in PrintErrMesg(), it went back to the bootloader, so I'm
> >>>>> sure this code path was reached. But with blexit() commented out, Xen
> >>>>> started correctly both with and without this patch... The branch I used
> >>>>> is here:
> >>>>> https://gitlab.com/xen-project/people/marmarek/xen/-/commits/automation-tests?ref_type=heads
> >>>>>
> >>>>> Are there some extra condition to reproduce the issue? Maybe it depends
> >>>>> on the compiler version? I guess I can try also on QEMU, but based on
> >>>>> the description, I would expect it to crash in any case.
> >>>>>
> >>>>
> >>>> Did you see the correct message in both cases?
> >>>> Did you use Grub or direct EFI?
> >>>>
> >>>> With Grub and without this patch you won't see the message, with grub
> >>>> with the patch you see the correct message.
> >>>
> >>> I did use grub, and I didn't see the message indeed.
> >>> But in the case it was supposed to crash (with added PrintErrMesg(),
> >>> commented out blexit and without your patch) it did _not_ crashed and
> >>> continued to normal boot. Is that #PF non-fatal here?
> >>>
> >>
> >> Hi,
> >>    I tried again with my test environment.
> >> Added the PrintErrMesg line before efi_arch_edd call, I got a #PF, in
> >> my case the system hangs. With the fix patch machine is rebooting and
> >> I can see the message in the logs.
> >> I'm trying with Xen starting inside Qemu, EFI firmware, xen.gz
> >> compiled as ELF file. Host system is an Ubuntu 22.04.5 LTS. Gcc is
> >> version 11.4.
> >
> > My test was wrong, commenting out blexit made "mesg" variable unused.
> > After fixing that, I can reproduce it on both QEMU and real hardware:
> > without your patch it crashes and with your patch it works just fine.
> > While there may be more places with similar issue, this patch clearly
> > improves the situation, so:
> >
> > Acked-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
>
> This had to be reverted, for breaking the build with old Clang. See the
> respective Matrix conversation.
>
> Jan
>

To sum up the failure is:

    clang: error: unknown argument: '-fno-jump-tables'

Frediano