v0.5.5: crash reports go to a dedicated CRASHLOG USB stick

The fault handler now writes crash.log to the volume labeled CRASHLOG
(label-selected via fat::crash_vol), falling back to slot 0.  On real
hardware the boot USB may be unwritable, so a second stick labeled
CRASHLOG survives the write; the shell auto-prints the previous crash
report from it at next boot (read_crash_log resolves /disk/<uid>/crash.log
through the FAT path, since Disk nodes hold no cached children).

- fat.rs: volume_label() + crash_vol() (no allocation)
- fs.rs: write_crash_log targets crash_vol(); read_crash_log prefers the
  CRASHLOG disk node; drop now-unused first_disk()
- mkdisk.sh: --label option
- test-fault.sh: boots with a dedicated CRASHLOG stick (mkfs -F 32 -n
  CRASHLOG), verifies crash.log lands there and NOT on the ESP

1 files changed, 17 insertions, 7 deletions

diff --git a/rsnix/src/gdt.rs b/rsnix/src/gdt.rs
index 514ed2e..f3d8b9a 100644
--- a/rsnix/src/gdt.rs
+++ b/rsnix/src/gdt.rs
@@ -116,12 +116,14 @@ impl Tss {
     }
 }
 
-/// 4 KiB page-aligned block for the IST stacks.
-#[repr(align(4096))]
-struct Page([u8; 4096]);
+/// IST stack size: the fault path (fault dump + best-effort crash-log
+/// FAT write) is deep, so each IST stack is 32 KiB, allocated from the
+/// heap at init.  They must NOT live in .bss: a 4 KiB .bss stack placed
+/// next to kernel statics silently overwrites them on overflow (the FAT
+/// ACTIVE_VOL was clobbered this way — a "no volume at slot 8" crash in
+/// the fault path).  The heap region is far from the kernel image.
+const IST_STACK_SIZE: usize = 32 * 1024;
 
-static mut PF_STACK: Page = Page([0; 4096]);
-static mut DF_STACK: Page = Page([0; 4096]);
 static mut TSS: Tss = Tss::zero();
 // GDT slots are 8 bytes except the TSS descriptor at index 63, which is
 // 16 bytes — total 64*8 + 16.
@@ -193,8 +195,16 @@ pub fn init() -> bool {
 
         // TSS descriptor (16 bytes) at index 63.
         let tss_base = core::ptr::addr_of_mut!(TSS) as usize as u64;
-        TSS.ist1 = core::ptr::addr_of_mut!(PF_STACK.0[0]) as usize as u64 + 4096;
-        TSS.ist2 = core::ptr::addr_of_mut!(DF_STACK.0[0]) as usize as u64 + 4096;
+        // IST stacks come from the heap (see IST_STACK_SIZE): 32 KiB each,
+        // well away from the kernel image/.bss so an overflowing fault
+        // path can't silently overwrite statics.
+        let pf = crate::memory::allocate(IST_STACK_SIZE);
+        let df = crate::memory::allocate(IST_STACK_SIZE);
+        if pf.is_null() || df.is_null() {
+            return false;
+        }
+        TSS.ist1 = pf as u64 + IST_STACK_SIZE as u64;
+        TSS.ist2 = df as u64 + IST_STACK_SIZE as u64;
         TSS.iomap_base = TSS_SIZE as u16; // no I/O bitmap
 
         let mut td = [0u8; 16];