Re: [Qemu-stable] [Qemu-devel] [PATCH v2] multiboot: Fix bss segment support

[Göran Weinholt]

Alexander Graf <address@hidden> writes:

> On 19.12.2011, at 23:01, Anthony Liguori wrote:
>
>     On 12/19/2011 11:35 AM, Alexander Graf wrote:
>
>         On 24.07.2011, at 17:55, Göran Weinholt wrote:
>
>             Multiboot images can specify a bss segment. The boot loader must
>             clear
>        
>             the memory of the bss and ensure that no modules or structures are
>        
>             allocated inside it. Several fields are provided in the Multiboot
>        
>             header that were previously not used properly. The header is now
>             used
>        
>             to determine how much data should be read from the image and how
>             much
>        
>             memory should be reserved to the bss segment.
>
>         This patch breaks the OSX booter:
>
>           http://people.exactcode.de/~rene/mac/boot
>
>     How is this licensed?  Is there source available?
>
> Yes, it's the XNU booter, but binaries are easier to debug when it comes to
> reading out the bootloader info:
>
>   http://tgwbd.org/darwin/boot.html
>
> I don't know if the binary above is 1:1 built from those sources, but it's
> definitely close enough.

I was unable to build that program from source, but I found a pre-built
binary that triggered the same bug. Please test the following patch.


Subject: [PATCH] multiboot: mh_load_end_addr and mh_bss_end_addr may be zero

There are two special cases in the address fields of the multiboot
format. If mh_load_end_addr is zero then the whole image file should
be loaded and if mh_bss_end_addr is zero then there is no bss segment.
With this change it is again possible to boot kernels where these
fields are zero.

Signed-off-by: Göran Weinholt <address@hidden>
---
 hw/multiboot.c |   13 ++++++++++++-
 1 files changed, 12 insertions(+), 1 deletions(-)

diff --git a/hw/multiboot.c b/hw/multiboot.c
index b4484a3..a43d56c 100644
--- a/hw/multiboot.c
+++ b/hw/multiboot.c
@@ -202,10 +202,21 @@ int load_multiboot(void *fw_cfg,
         uint32_t mh_bss_end_addr = ldl_p(header+i+24);
         mh_load_addr = ldl_p(header+i+16);
         uint32_t mb_kernel_text_offset = i - (mh_header_addr - mh_load_addr);
-        uint32_t mb_load_size = mh_load_end_addr - mh_load_addr;
+        uint32_t mb_load_size;
+
+        /* A load end address of zero indicates that the whole file
+         * should be loaded. */
+        if (!mh_load_end_addr)
+            mh_load_end_addr = kernel_file_size + mh_load_addr;
+
+        /* A bss end address of zero indicates that there is no bss
+         * segment. */
+        if (!mh_bss_end_addr)
+            mh_bss_end_addr = mh_load_end_addr;
 
         mh_entry_addr = ldl_p(header+i+28);
         mb_kernel_size = mh_bss_end_addr - mh_load_addr;
+        mb_load_size = mh_load_end_addr - mh_load_addr;
 
         /* Valid if mh_flags sets MULTIBOOT_HEADER_HAS_VBE.
         uint32_t mh_mode_type = ldl_p(header+i+32);
-- 
1.7.2.5


-- 
Göran Weinholt <address@hidden>

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