On 2020-09-25 10:34, Michael Kerrisk (man-pages) wrote:
Hi Alex,
On 9/25/20 9:31 AM, Alejandro Colomar wrote:
Signed-off-by: Alejandro Colomar <colomar.6.4.3@xxxxxxxxx>
---
man2/seccomp.2 | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/man2/seccomp.2 b/man2/seccomp.2
index 58033da1c..d6b856c32 100644
--- a/man2/seccomp.2
+++ b/man2/seccomp.2
@@ -1101,7 +1101,7 @@ install_filter(int syscall_nr, int t_arch, int f_errno)
};
struct sock_fprog prog = {
- .len = (unsigned short) (sizeof(filter) / sizeof(filter[0])),
+ .len = sizeof(filter) / sizeof(filter[0]),
.filter = filter,
};
I have a small doubt about this change. With the change,
there are no compilation warnings.
But, if we change the code to something slightly different:
[[
size_t x = (sizeof(filter) / sizeof(filter[0]));
struct sock_fprog prog = {
.len = x,
.filter = filter,
};
]]
The "cc -Wconversion" gives us the following warning:
warning: conversion from ‘size_t’ {aka ‘long unsigned int’}
to ‘short unsigned int’ may change value
Presumably we don't get a warning for an assignment of the form
.len = (sizeof(filter) / sizeof(filter[0]))
because the compiler is smart enough to work out that the
value of the constant expression is within the range of
"unsigned short".
Your thoughts?
Hi Michael,
I'd say that the cast doesn't fix any problems at all. It silences a
valid warning, and I'd use a pragma for that (to be more explicit about
the intention of silencing a warning) if I do want -Wconversion enabled
(which usually I don't want, because it's too noisy) and I'm sure that
this won't overflow. I'd limit the use casts to only when I *really*
need to.
I guess that if you enable -O3, the warning will vanish again because
the compiler will optimize away 'x' (but I didn't test).
Thanks,
Alex
Thanks,
Michael
