Eric and Hugh have reported instant reboot due to my recent changes in
decompression code.

The root cause is that I didn't realize that we need to adjust GOT to be
able to run C code that early.

The problem is only visible with an older toolchain. Binutils >= 2.24 is
able to eliminate GOT references by replacing them with RIP-relative
address loads[1].

We need to adjust GOT two times:
 - before calling paging_prepare() using the initial load address
 - before calling C code from the relocated kernel

[1] 
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commitdiff;h=80d873266dec

Signed-off-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
Fixes: 194a9749c73d ("x86/boot/compressed/64: Handle 5-level paging boot if 
kernel is above 4G")
Reported-by: Eric Dumazet <eric.duma...@gmail.com>
Reported-by: Hugh Dickins <hu...@google.com>
---
 arch/x86/boot/compressed/head_64.S | 68 ++++++++++++++++++++++++------
 1 file changed, 55 insertions(+), 13 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S 
b/arch/x86/boot/compressed/head_64.S
index fca012baba19..d17af6a4bfc9 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -305,6 +305,25 @@ ENTRY(startup_64)
        /* Set up the stack */
        leaq    boot_stack_end(%rbx), %rsp
 
+       /*
+        * paging_prepare() and cleanup_trampoline() below can have GOT
+        * references. Adjust the table with address we are running at.
+        *
+        * Zero RAX for adjust_got: the GOT was not adjusted before;
+        * there's no adjustment to undo.
+        */
+       xorq    %rax, %rax
+
+       /*
+        * Calculate the address the binary is loaded at and use it as
+        * a GOT adjustment.
+        */
+       call    1f
+1:     popq    %rdi
+       subq    $1b, %rdi
+
+       call    adjust_got
+
        /*
         * At this point we are in long mode with 4-level paging enabled,
         * but we might want to enable 5-level paging or vice versa.
@@ -381,6 +400,21 @@ trampoline_return:
        pushq   $0
        popfq
 
+       /*
+        * Previously we've adjusted the GOT with address the binary was
+        * loaded at. Now we need to re-adjust for relocation address.
+        *
+        * Calculate the address the binary is loaded at, so that we can
+        * undo the previous GOT adjustment.
+        */
+       call    1f
+1:     popq    %rax
+       subq    $1b, %rax
+
+       /* The new adjustment is the relocation address */
+       movq    %rbx, %rdi
+       call    adjust_got
+
 /*
  * Copy the compressed kernel to the end of our buffer
  * where decompression in place becomes safe.
@@ -481,19 +515,6 @@ relocated:
        shrq    $3, %rcx
        rep     stosq
 
-/*
- * Adjust our own GOT
- */
-       leaq    _got(%rip), %rdx
-       leaq    _egot(%rip), %rcx
-1:
-       cmpq    %rcx, %rdx
-       jae     2f
-       addq    %rbx, (%rdx)
-       addq    $8, %rdx
-       jmp     1b
-2:
-       
 /*
  * Do the extraction, and jump to the new kernel..
  */
@@ -512,6 +533,27 @@ relocated:
  */
        jmp     *%rax
 
+/*
+ * Adjust the global offset table
+ *
+ * RAX is the previous adjustment of the table to undo (use 0 if it's the
+ * first time we touch GOT).
+ * RDI is the new adjustment to apply.
+ */
+adjust_got:
+       /* Walk through the GOT adding the address to the entries */
+       leaq    _got(%rip), %rdx
+       leaq    _egot(%rip), %rcx
+1:
+       cmpq    %rcx, %rdx
+       jae     2f
+       subq    %rax, (%rdx)    /* Undo previous adjustment */
+       addq    %rdi, (%rdx)    /* Apply the new adjustment */
+       addq    $8, %rdx
+       jmp     1b
+2:
+       ret
+
        .code32
 /*
  * This is the 32-bit trampoline that will be copied over to low memory.
-- 
2.17.0

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