Re: [PATCH v5] Avoid crash calling PrintErrMesg from efi_multiboot2

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/01/2025 10:14 am, Frediano Ziglio wrote:
> Although code is compiled with -fpic option data is not position
> independent.

This doesn't parse.  ITYM "Although the code is compiled with -fpic,
pointers in data are 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.

"efi_multiboot2()" to highlight that you're naming a function.

>
> 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>

00d5d5ce23e6 needs to be a second fixes tag then too.

In hindsight, it was a naive fix.  But, by reverting this part of it,
you're reintroducing a build breakage on Clang.

I wrote that construct because it is what Clang-3.8 generated, albeit
with the proper __initconstrel attribute to make the
SPEICAL_DATA_SECTIONS check be happy.

Anyway, see:
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=x86/urgent&id=1105ab42a84bc11c62597005f78ccad2434fbd66

This needs to be paired with an unconditional -fno-jump-tables covering
any TU which the MB2+EFI path executes at the wrong address.

~Andrew