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Re: [Xen-devel] [PATCH] xen/build-id: Fix xen_build_id_check() to be robust against malformed notes
- To: Wei Liu <wei.liu2@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 2 Jan 2019 12:01:10 +0000
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- Cc: Julien Grall <julien.grall@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
- Delivery-date: Wed, 02 Jan 2019 12:01:18 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 02/01/2019 10:43, Wei Liu wrote:
> On Mon, Dec 31, 2018 at 05:34:25PM +0000, Andrew Cooper wrote:
>> A NT_GNU_BUILD_ID with namesz longer than 4 will cause the strncmp() to use
>> bytes in adjacent stringtable entries.
>>
>> Instead, check for namesz exactly equal to 4, and use memcmp() with an
>> explicit size.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>> CC: Julien Grall <julien.grall@xxxxxxx>
>>
>> Noticed while auditing Xen's use of strncmp() for the command line patch.
>> ---
>> xen/common/version.c | 6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/xen/common/version.c b/xen/common/version.c
>> index 223cb52..1df7e78 100644
>> --- a/xen/common/version.c
>> +++ b/xen/common/version.c
>> @@ -97,17 +97,17 @@ int xen_build_id_check(const Elf_Note *n, unsigned int
>> n_sz,
>> if ( NT_GNU_BUILD_ID != n->type )
>> return -ENODATA;
>>
>> - if ( n->namesz + n->descsz < n->namesz )
>> + if ( n->namesz != 4 /* GNU\0 */)
>> return -EINVAL;
>>
>> - if ( n->namesz < 4 /* GNU\0 */)
>> + if ( n->namesz + n->descsz < n->namesz )
> The reordering of two predicates doesn't seem to serve any particular
> purpose? You could've just changed "<" to "!=" for less code churn?
Logically, the != 4 check should be ahead of the truncation check, but
yes - patch did collapse it into something rather harder to read in this
case.
>
>> return -EINVAL;
>>
>> if ( n->namesz + n->descsz > n_sz - sizeof(*n) )
>> return -EINVAL;
>>
>> /* Sanity check, name should be "GNU" for ld-generated build-id. */
>> - if ( strncmp(ELFNOTE_NAME(n), "GNU", n->namesz) != 0 )
>> + if ( memcmp(ELFNOTE_NAME(n), "GNU", 4) != 0 )
> OOI what is the advantage of memcmp compared to strncmp?
Erm. I suppose in this exact case, personal preference.
~Andrew
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