loader: Fix ELF relocation logic for Linux v6.17+ startup code layout (#2298)

After Linux v6.17, the x86_64 kernel startup code moved from .head.text
to .init.text. This moves the startup code to an address, higher than
the actual physical load address. Update the loader to check the
lowest program header address for relocation, not just the entry point,
to handle both old and new kernels.

This issue was identified when latest upstream kernel is built with
latest openvmm.
Compilation error:
Error: underhill kernel loader

Caused by:
    0: kernel load error
    1: elf loader error
2: invalid ELF program header memory offset 134217728, below start
136314880

Tested the change, for bootup, both with current mainline and latest
upstream kernel.

Co-authored-by: Naman Jain <namjain@microsoft.com>
Co-authored-by: GitHub Copilot <copilot@microsoft.com>

Author: namancse

vm/loader/src/elf.rs

@@ -133,9 +133,29 @@ where
     }
 
     let e_entry = ehdr.e_entry.get(LE);
-    let load_offset = if assume_pic && e_entry < start_address {
-        // The kernel is assumed to contain PIC
-        start_address + load_offset
+    let phdrs = ehdr
+        .program_headers(LE, &reader)
+        .map_err(Error::InvalidProgramHeader)?;
+
+    // For PIC kernels, calculate load offset by checking lowest paddr in program headers.
+    // If it is below start_address, relocate kernel upward. Handles both:
+    // - Old kernels (< v6.17): startup code is in .head.text at start of .text, low entry point,
+    //   and matches physical load address
+    // - New kernels (≥ v6.17): startup code is in .init.text (commit: "x86/boot: Move startup code out of __head section"),
+    //   high entry point but low physical load address
+    let load_offset = if assume_pic {
+        let mut lowest_paddr = u64::MAX;
+        for phdr in phdrs {
+            if phdr.p_type.get(LE) == elf::PT_LOAD {
+                let p_paddr = phdr.p_paddr.get(LE);
+                lowest_paddr = lowest_paddr.min(p_paddr);
+            }
+        }
+        if lowest_paddr < start_address {
+            start_address - lowest_paddr + load_offset
+        } else {
+            load_offset
+        }
     } else {
         load_offset
     };
@@ -155,10 +175,6 @@ where
         });
     }
 
-    let phdrs = ehdr
-        .program_headers(LE, &reader)
-        .map_err(Error::InvalidProgramHeader)?;
-
     // The first pass on the sections provides the layout data
     let (lowest_addr, last_offset, reloc_bias) = {
         let mut lowest_addr = u64::MAX;