Re: [PATCH] x86/boot/compressed/64: Fix 'may be used uninitialized' issue

From: Zhenzhong Duan
Date: Mon Sep 02 2019 - 05:41:05 EST

On 2019/9/2 17:35, Borislav Petkov wrote:
On Mon, Sep 02, 2019 at 05:30:24PM +0800, Zhenzhong Duan wrote:
Builds kernel with "make bzImage EXTRA_CFLAGS=-Wall", gcc complains:

arch/x86/boot/compressed/pgtable_64.c: In function 'paging_prepare':
arch/x86/boot/compressed/pgtable_64.c:92:7: warning: 'new' may be used uninitialized in this function [-Wmaybe-uninitialized]
new = round_down(new, PAGE_SIZE);

In theory a random value of variable new may pass all the check and be
assigned to bios_start, fixing it by assigning it an initial value.

Fixes: 0a46fff2f910 ("x86/boot/compressed/64: Fix boot on machines with broken E820 table")
Signed-off-by: Zhenzhong Duan<zhenzhong.duan@xxxxxxxxxx>
Cc: Kirill A. Shutemov<kirill@xxxxxxxxxxxxx>
Cc: Thomas Gleixner<tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar<mingo@xxxxxxxxxx>
Cc: Borislav Petkov<bp@xxxxxxxxx>
Cc: "H. Peter Anvin"<hpa@xxxxxxxxx>
Cc: x86-ml<x86@xxxxxxxxxx>
arch/x86/boot/compressed/pgtable_64.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index 2faddeb..c886269 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -72,7 +72,7 @@ static unsigned long find_trampoline_placement(void)
/* Find the first usable memory region under bios_start. */
for (i = boot_params->e820_entries - 1; i >= 0; i--) {
- unsigned long new;
+ unsigned long new = bios_start;
entry = &boot_params->e820_table[i];

In the future, use tip/master when preparing fixes because it has the
latest state of the x86 tree and you won't have to duplicate effort,
like in this case.

Ok, I see. Thanks for your suggestion.