[v3,1/5] qemu/compiler: Define QEMU_NONSTRING
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Message ID 20181218175122.3229-2-philmd@redhat.com
State New
Headers show
Series
  • Fix strncpy() warnings for GCC8 new -Wstringop-truncation
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Commit Message

Philippe Mathieu-Daudé Dec. 18, 2018, 5:51 p.m. UTC
GCC 8 introduced the -Wstringop-truncation checker to detect truncation by
the strncat and strncpy functions (closely related to -Wstringop-overflow,
which detect buffer overflow by string-modifying functions declared in
<string.h>).

Add the QEMU_NONSTRING macro which checks if the compiler supports this
attribute.

From the GCC manual [*]:

  The nonstring variable attribute specifies that an object or member
  declaration with type array of char, signed char, or unsigned char,
  or pointer to such a type is intended to store character arrays that
  do not necessarily contain a terminating NUL. This is useful in detecting
  uses of such arrays or pointers with functions that expect NUL-terminated
  strings, and to avoid warnings when such an array or pointer is used as
  an argument to a bounded string manipulation function such as strncpy.

[*] https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 include/qemu/compiler.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

Comments

Eric Blake Dec. 18, 2018, 6:29 p.m. UTC | #1
On 12/18/18 11:51 AM, Philippe Mathieu-Daudé wrote:
> GCC 8 introduced the -Wstringop-truncation checker to detect truncation by
> the strncat and strncpy functions (closely related to -Wstringop-overflow,
> which detect buffer overflow by string-modifying functions declared in
> <string.h>).

This paragraph talks about a new warning checker, but makes no mention 
of an attribute.

> 
> Add the QEMU_NONSTRING macro which checks if the compiler supports this
> attribute.

Thus, "this attribute" has no antecedent; did you forget to add a 
sentence to the previous paragraph, or maybe put the mention of adding 
QEMU_NONSTRING after...

> 
>>From the GCC manual [*]:
> 
>    The nonstring variable attribute specifies that an object or member
>    declaration with type array of char, signed char, or unsigned char,
>    or pointer to such a type is intended to store character arrays that
>    do not necessarily contain a terminating NUL. This is useful in detecting
>    uses of such arrays or pointers with functions that expect NUL-terminated
>    strings, and to avoid warnings when such an array or pointer is used as
>    an argument to a bounded string manipulation function such as strncpy.

...the explanation of how the attribute was added in tandem with the new 
warning checker for silencing specific instances of the warning?

> 
> [*] https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute
> 
> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>   include/qemu/compiler.h | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
> 

Reviewed-by: Eric Blake <eblake@redhat.com>
Philippe Mathieu-Daudé Dec. 18, 2018, 7:28 p.m. UTC | #2
On 12/18/18 7:29 PM, Eric Blake wrote:
> On 12/18/18 11:51 AM, Philippe Mathieu-Daudé wrote:
>> GCC 8 introduced the -Wstringop-truncation checker to detect
>> truncation by
>> the strncat and strncpy functions (closely related to
>> -Wstringop-overflow,
>> which detect buffer overflow by string-modifying functions declared in
>> <string.h>).
> 
> This paragraph talks about a new warning checker, but makes no mention
> of an attribute.
> 
>>
>> Add the QEMU_NONSTRING macro which checks if the compiler supports this
>> attribute.
> 
> Thus, "this attribute" has no antecedent; did you forget to add a
> sentence to the previous paragraph, or maybe put the mention of adding
> QEMU_NONSTRING after...
> 
>>
>>> From the GCC manual [*]:
>>
>>    The nonstring variable attribute specifies that an object or member
>>    declaration with type array of char, signed char, or unsigned char,
>>    or pointer to such a type is intended to store character arrays that
>>    do not necessarily contain a terminating NUL. This is useful in
>> detecting
>>    uses of such arrays or pointers with functions that expect
>> NUL-terminated
>>    strings, and to avoid warnings when such an array or pointer is
>> used as
>>    an argument to a bounded string manipulation function such as strncpy.
> 
> ...the explanation of how the attribute was added in tandem with the new
> warning checker for silencing specific instances of the warning?

Yes... I will rewrite this.

> 
>>
>> [*]
>> https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute
>>
>>
>> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>>   include/qemu/compiler.h | 15 +++++++++++++++
>>   1 file changed, 15 insertions(+)
>>
> 
> Reviewed-by: Eric Blake <eblake@redhat.com>

Thanks!

Patch
diff mbox series

diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
index 261842beae..2d8f507c73 100644
--- a/include/qemu/compiler.h
+++ b/include/qemu/compiler.h
@@ -151,6 +151,21 @@ 
 # define QEMU_ERROR(X)
 #endif
 
+/*
+ * The nonstring variable attribute specifies that an object or member
+ * declaration with type array of char or pointer to char is intended
+ * to store character arrays that do not necessarily contain a terminating
+ * NUL character. This is useful in detecting uses of such arrays or pointers
+ * with functions that expect NUL-terminated strings, and to avoid warnings
+ * when such an array or pointer is used as an argument to a bounded string
+ * manipulation function such as strncpy.
+ */
+#if __has_attribute(nonstring)
+# define QEMU_NONSTRING __attribute__((nonstring))
+#else
+# define QEMU_NONSTRING
+#endif
+
 /* Implement C11 _Generic via GCC builtins.  Example:
  *
  *    QEMU_GENERIC(x, (float, sinf), (long double, sinl), sin) (x)