Re: [RFC PATCH 2/2] linux-user/mmap: Fix Clang 'type-limit-compare' warning

[Eric Blake]

On 5/3/20 6:32 AM, Philippe Mathieu-Daudé wrote:
> When building with Clang 10 on Fedora 32, we get:
>
>      CC      linux-user/mmap.o
>    linux-user/mmap.c:720:49: error: result of comparison 'unsigned long' > 
> 18446744073709551615 is always false [-Werror,-Wtautological-type-limit-compare]
>            if ((unsigned long)host_addr + new_size > (abi_ulong)-1) {
>                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~
>
> Fix by restricting the check for when target sizeof(abi_ulong) is
> smaller than target sizeof(unsigned long).
>
> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> ---
>   linux-user/mmap.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/linux-user/mmap.c b/linux-user/mmap.c
> index e378033797..b14652d894 100644
> --- a/linux-user/mmap.c
> +++ b/linux-user/mmap.c
> @@ -714,6 +714,7 @@ abi_long target_mremap(abi_ulong old_addr, abi_ulong 
> old_size,
>               errno = ENOMEM;
>               host_addr = MAP_FAILED;
>           }
> +#if TARGET_ABI_BITS < TARGET_LONG_BITS
>           /* Check if address fits target address space */
>           if ((unsigned long)host_addr + new_size > (abi_ulong)-1) {

Instead of using #if, the following suffices to shut up clang:

diff --git c/linux-user/mmap.c w/linux-user/mmap.c
index e37803379747..8d9ba201625d 100644
--- c/linux-user/mmap.c
+++ w/linux-user/mmap.c
@@ -715,7 +715,7 @@ abi_long target_mremap(abi_ulong old_addr, abi_ulong 
old_size,
             host_addr = MAP_FAILED;
         }
         /* Check if address fits target address space */
-        if ((unsigned long)host_addr + new_size > (abi_ulong)-1) {
+        if ((unsigned long)host_addr > (abi_ulong)-1 - new_size) {
             /* Revert mremap() changes */
             host_addr = mremap(g2h(old_addr), new_size, old_size, flags);
             errno = ENOMEM;


That is, it is no longer a tautological type compare if you commute the 
operations so that neither side is a compile-time constant.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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