Re: [PATCH] target/s390x/arch_dump: Fixes for the name field in the PT_NOTE section

[Thomas Huth <thuth@redhat.com>]

On 04/02/2021 14.01, Christian Borntraeger wrote:
> 
> 
> On 03.02.21 10:44, Thomas Huth wrote:
>> According to the "ELF-64 Object File Format" specification:
>>
>> "The first word in the entry, namesz, identifies the length, in
>>   bytes, of a name identifying the entry’s owner or originator. The name field
>>   contains a null-terminated string, with padding as necessary to ensure 8-
>>   byte alignment for the descriptor field. The length does not include the
>>   terminating null or the padding."
>>
>> So we should not include the terminating NUL in the length field here.
>>
>> Also there is a compiler warning with GCC 9.3 when compiling with
>> the -fsanitize=thread compiler flag:
>>
>>   In function 'strncpy',
>>      inlined from 's390x_write_elf64_notes' at ../target/s390x/arch_dump.c:219:9:
>>   /usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: error:
>>    '__builtin_strncpy' specified bound 8 equals destination size
>>    [-Werror=stringop-truncation]
>>
>> Since the name should always be NUL-terminated, we can simply decrease
>> the size of the strncpy by one here to silence this warning. And while
>> we're at it, also add an assert() to make sure that the provided names
>> always fit the size field (which is fine for the current callers, the
>> function is called once with "CORE" and once with "LINUX" as a name).
>>
>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>> ---
>>   The ELF-64 spec can be found here, for example:
>>   https://uclibc.org/docs/elf-64-gen.pdf
>>
>>   Here's a CI run with the compiler warning:
>>   https://gitlab.com/huth/qemu/-/jobs/1003508341#L1248
>>
>>   target/s390x/arch_dump.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/target/s390x/arch_dump.c b/target/s390x/arch_dump.c
>> index 50fa0ae4b6..20c3a09707 100644
>> --- a/target/s390x/arch_dump.c
>> +++ b/target/s390x/arch_dump.c
>> @@ -212,11 +212,13 @@ static int s390x_write_elf64_notes(const char *note_name,
>>       int note_size;
>>       int ret = -1;
>>   
>> +    assert(strlen(note_name) < sizeof(note.name));
>> +
>>       for (nf = funcs; nf->note_contents_func; nf++) {
>>           memset(&note, 0, sizeof(note));
>> -        note.hdr.n_namesz = cpu_to_be32(strlen(note_name) + 1);
>> +        note.hdr.n_namesz = cpu_to_be32(strlen(note_name));
>>           note.hdr.n_descsz = cpu_to_be32(nf->contents_size);
>> -        strncpy(note.name, note_name, sizeof(note.name));
>> +        strncpy(note.name, note_name, sizeof(note.name) - 1);
> 
> This kind of feels wrong. With 8 bytes of note.name, we should be able to store "Test123" + the final \0.
> Now we tell strncpy to stop at 7. Which means that for Test123 we would NOT copy the final \0.

Yes, but there is a memset(&note, 0, sizeof(note)) at the beginning of the 
for-loop, so the 8th byte should always be set to 0 anyway. But if you 
prefer, I can also simply use g_strlcpy() instead...?

  Thomas