[reactos] 01/02: [FREELDR] Fix displayed information in the Exception BSOD. CORE-16748

[Hermès Bélusca-Maïto]

https://git.reactos.org/?p=reactos.git;a=commitdiff;h=2c9dbacebbee6b53bf331…

commit 2c9dbacebbee6b53bf331e032d9f2af4e4f828eb
Author:     Hermès Bélusca-Maïto <hermes.belusca-maito(a)reactos.org>
AuthorDate: Sat Jan 1 02:24:52 2022 +0100
Commit:     Hermès Bélusca-Maïto <hermes.belusca-maito(a)reactos.org>
CommitDate: Sat Jan 1 05:04:22 2022 +0100

    [FREELDR] Fix displayed information in the Exception BSOD.
    CORE-16748
    
    - Display the correct TR register value.
    
    - Ensure that the x86 segment register values displayed are really
      2-byte long.
    
    Segment registers are intrinsically 16 bits. Even if the x86
    KTRAP_FRAME structure stores them as ULONG, only their lower 16 bits
    are initialized. We thus cast them to USHORT before display.
    
    These segment registers are saved in a stack-based KTRAP_FRAME by the
    CPU trap mechanism (for SS), and by 'push CS' etc. instructions for
    the others, and from Intel documentation, we know that:
    "
    If the source operand is a segment register (16 bits) and the operand
    size is 64-bits, a zero-extended value is pushed on the stack; if the
    operand size is 32-bits, either a zero-extended value is pushed on the
    stack or the segment selector is written on the stack using a 16-bit
    move. For the last case, all recent Core and Atom processors perform
    a 16-bit move, leaving the upper portion of the stack location unmodified.
    "
    So it may happen, when using the push, that either they get zero-extended,
    or garbage gets stored in the higher bits, and these need to be trimmed.
---
 boot/freeldr/freeldr/arch/i386/i386bug.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/boot/freeldr/freeldr/arch/i386/i386bug.c
b/boot/freeldr/freeldr/arch/i386/i386bug.c
index f94639650f4..908ad46950c 100644
--- a/boot/freeldr/freeldr/arch/i386/i386bug.c
+++ b/boot/freeldr/freeldr/arch/i386/i386bug.c
@@ -140,18 +140,22 @@ i386PrintExceptionText(ULONG TrapIndex, PKTRAP_FRAME TrapFrame,
PKSPECIAL_REGIST
               TrapFrame->Dr6);
     PrintText("                                                               DR7:
%.8lx\n\n",
               TrapFrame->Dr7);
+
+    /* NOTE: Segment registers are intrinsically 16 bits. Even if the x86
+     * KTRAP_FRAME structure stores them as ULONG, only their lower 16 bits
+     * are initialized. We thus cast them to USHORT before display. */
     PrintText("CS: %.4lx        EIP: %.8lx\n",
-              TrapFrame->SegCs, TrapFrame->Eip);
+              (USHORT)TrapFrame->SegCs, TrapFrame->Eip);
     PrintText("DS: %.4lx        ERROR CODE: %.8lx\n",
-              TrapFrame->SegDs, TrapFrame->ErrCode);
+              (USHORT)TrapFrame->SegDs, TrapFrame->ErrCode);
     PrintText("ES: %.4lx        EFLAGS: %.8lx\n",
-              TrapFrame->SegEs, TrapFrame->EFlags);
+              (USHORT)TrapFrame->SegEs, TrapFrame->EFlags);
     PrintText("FS: %.4lx        GDTR Base: %.8lx Limit: %.4x\n",
-              TrapFrame->SegFs, Special->Gdtr.Base, Special->Gdtr.Limit);
+              (USHORT)TrapFrame->SegFs, Special->Gdtr.Base,
Special->Gdtr.Limit);
     PrintText("GS: %.4lx        IDTR Base: %.8lx Limit: %.4x\n",
-              TrapFrame->SegGs, Special->Idtr.Base, Special->Idtr.Limit);
+              (USHORT)TrapFrame->SegGs, Special->Idtr.Base,
Special->Idtr.Limit);
     PrintText("SS: %.4lx        LDTR: %.4lx TR: %.4lx\n\n",
-              TrapFrame->HardwareSegSs, Special->Ldtr, Special->Idtr.Limit);
+              (USHORT)TrapFrame->HardwareSegSs, Special->Ldtr, Special->Tr);
 
     i386PrintFrames(TrapFrame);                        // Display frames
     InstructionPointer = (PUCHAR)TrapFrame->Eip;
@@ -176,7 +180,7 @@ i386PrintExceptionText(ULONG TrapIndex, PKTRAP_FRAME TrapFrame,
PKSPECIAL_REGIST
     PrintText("GS: %.4lx        IDTR Base: %.8lx Limit: %.4x\n",
               TrapFrame->SegGs, Special->Idtr.Base, Special->Idtr.Limit);
     PrintText("SS: %.4lx        LDTR: %.4lx TR: %.4lx\n\n",
-              TrapFrame->SegSs, Special->Ldtr, Special->Idtr.Limit);
+              TrapFrame->SegSs, Special->Ldtr, Special->Tr);
     InstructionPointer = (PUCHAR)TrapFrame->Rip;
 #endif
     PrintText("\nInstruction stream: %.2x %.2x %.2x %.2x %.2x %.2x %.2x %.2x
\n",