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Re: [Xen-devel] [PATCH v6 05/26] xen/arm: check for multiboot nodes only under /chosen

Hi Stefano,

On 11/9/18 9:38 PM, Stefano Stabellini wrote:
On Fri, 9 Nov 2018, Julien Grall wrote:

On 02/11/2018 23:44, Stefano Stabellini wrote:
Make sure to only look for multiboot compatible nodes only under
/chosen, not under any other paths (depth <= 3).

Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>


Changes in v6:
- do not proceed if fdt_get_path returns error != -FDT_ERR_NOSPACE
- remove sizeof, use hardcoded value

Changes in v5:
- add patch
- add check on return value of fdt_get_path
   xen/arch/arm/bootfdt.c | 14 +++++++++++---
   1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/bootfdt.c b/xen/arch/arm/bootfdt.c
index 8eba42c..a42fe87 100644
--- a/xen/arch/arm/bootfdt.c
+++ b/xen/arch/arm/bootfdt.c
@@ -173,7 +173,15 @@ static void __init process_multiboot_node(const void
*fdt, int node,
       bootmodule_kind kind;
       paddr_t start, size;
       const char *cmdline;
-    int len;
+    int len = 8; /* sizeof "/chosen" */
+    char path[8];
+    int ret;
+    /* Check that the node is under "chosen" */
+    ret = fdt_get_path(fdt, node, path, len);
+    if ( (ret != 0 && ret != -FDT_ERR_NOSPACE) ||

I don't understand why you specifically check for -FDT_ERR_NOSPACE here.

Looking at fdt_get_path(...) there are case where the function may return
-FDT_ERR_NOSPACE yet not filling up path. So you would end up to compare

I am explicitely checking for -FDT_ERR_NOSPACE because it is a valid
condition in this case: -FDT_ERR_NOSPACE is returned where `path' is not
big enough to contain the full device tree path. It is OK and expected,
given that path is only 8 chars long. So, in case of -FDT_ERR_NOSPACE,
we should continue and do the comparison with "/chosen". For other
errors we should return.

Let's take an example with a path called /deadbeef. This will not hold in the variable path. Do you agree that fdt_get_path will return -FDT_ERR_NO_SPACE in that case?

AFAIU the function fdt_get_path, the buffer will contain the character
/ followed by garbage as '\0' is only added in successful path.

This also fit with the description of fdt_get_path when -FDT_ERR_NO_SPACE. It does not promise you the buffer will contain anything. It only tells you that the path on the given node will not fit in the buffer.

So I still don't think you can assume the behavior you described above.


Julien Grall

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