Re: [PATCH v1 2/2] curl_off_t xcurl_off_t is not portable
On Fri, Oct 26, 2018 at 11:48:38AM +0900, Junio C Hamano wrote:
> tbo...@web.de writes:
>
> > From: Torsten Bögershausen
>
> > Subject: Re: [PATCH v1 2/2] curl_off_t xcurl_off_t is not portable
>
> That title is misleading; it sounded as if the are these two
> typedefs and they do not work correctly on some platforms, but that
> is not what you are doing with the patch.
OK.
>
> > Comparing signed and unsigned values is not always portable.
>
> Is that what the compiler is complaining about? There is this bit
> in git-compat-util.h:
No, not that either, see below.
>
> /*
> * Signed integer overflow is undefined in C, so here's a helper macro
> * to detect if the sum of two integers will overflow.
> *
> * Requires: a >= 0, typeof(a) equals typeof(b)
> */
> #define signed_add_overflows(a, b) \
> ((b) > maximum_signed_value_of_type(a) - (a))
>
> which is designed to be fed signed a and signed b. The macro is
> used in packfile codepaths to compare int, off_t, etc..
>
> So the statement may be true but it does not seem to have much to do
> with the problem you are seeing with maximum_signed_value_of_type().
>
> > When setting
> > DEVELOPER = 1
> > DEVOPTS = extra-all
> >
> > "gcc (Raspbian 6.3.0-18+rpi1+deb9u1) 6.3.0 20170516" errors out with
> > "comparison is always false due to limited range of data type"
> > "[-Werror=type-limits]"
>
> Then this sounds a bit different from "comparison between signed
> ssize_t len and unsigned maximum_signed_value_of_type() is bad".
> Isn't it saying that "No matter how big you make len, you can never
> go beyond maximum_signed_value_of_type(curl_off_t)"?
I digged a little bit deeper into the raspi, and this is what I find
under
/usr/include/arm-linux-gnueabihf/curl
curlbuild.h:#define CURL_TYPEOF_CURL_OFF_T int64_t
curlbuild.h:typedef CURL_TYPEOF_CURL_OFF_T curl_off_t;
>
> > diff --git a/remote-curl.c b/remote-curl.c
> > index 762a55a75f..c89fd6d1c3 100644
> > --- a/remote-curl.c
> > +++ b/remote-curl.c
> > @@ -618,9 +618,10 @@ static int probe_rpc(struct rpc_state *rpc, struct
> > slot_results *results)
> > }
> >
> > static curl_off_t xcurl_off_t(ssize_t len) {
> > - if (len > maximum_signed_value_of_type(curl_off_t))
>
> Is the issue that len is signed and maximum_signed_value_of_type()
> gives an unsigned value, and these two are compared? As we saw
> earlier, signed_add_overflows() is another example that wants a
> mixed comparison.
>
> I am just wondering if casting len to uintmax_t before comparing
> with maximum_signed_value_of_type() is a simpler solution that can
> safely be cargo-culted to other places without much thinking.
I don't know.
Since ssize_t is 32 bit on the raspi, and curl_off_t is 64 bit,
the test seems not to be needed at all ;-)
I don't know if it makes sense to stop thinking here and if
casting to uintmax_t is the right solution here.
And, I like the easy-to-read xsize_t, which is safe and warm.
Agreed that the commit message is wrong.
I would like to keep the xsize_t aproach, are there more thoughts ?
>
> "git grep maximum_signed_value_of_type" reports a handful
> comparisons in vcs-svn/, all of which does
>
> if (var > maximum_signed_value_of_type(off_t))
>
> with var of type uintmax_t, which sounds like a sane thing to do.
>
> Thanks.
>
> > + curl_off_t size = (curl_off_t) len;
> > + if (len != (ssize_t) size)
> > die("cannot handle pushes this big");
> > - return (curl_off_t) len;
> > + return size;
> > }
>
Re: [PATCH v1 2/2] curl_off_t xcurl_off_t is not portable
tbo...@web.de writes:
> From: Torsten Bögershausen
> Subject: Re: [PATCH v1 2/2] curl_off_t xcurl_off_t is not portable
That title is misleading; it sounded as if the are these two
typedefs and they do not work correctly on some platforms, but that
is not what you are doing with the patch.
> Comparing signed and unsigned values is not always portable.
Is that what the compiler is complaining about? There is this bit
in git-compat-util.h:
/*
* Signed integer overflow is undefined in C, so here's a helper macro
* to detect if the sum of two integers will overflow.
*
* Requires: a >= 0, typeof(a) equals typeof(b)
*/
#define signed_add_overflows(a, b) \
((b) > maximum_signed_value_of_type(a) - (a))
which is designed to be fed signed a and signed b. The macro is
used in packfile codepaths to compare int, off_t, etc..
So the statement may be true but it does not seem to have much to do
with the problem you are seeing with maximum_signed_value_of_type().
> When setting
> DEVELOPER = 1
> DEVOPTS = extra-all
>
> "gcc (Raspbian 6.3.0-18+rpi1+deb9u1) 6.3.0 20170516" errors out with
> "comparison is always false due to limited range of data type"
> "[-Werror=type-limits]"
Then this sounds a bit different from "comparison between signed
ssize_t len and unsigned maximum_signed_value_of_type() is bad".
Isn't it saying that "No matter how big you make len, you can never
go beyond maximum_signed_value_of_type(curl_off_t)"?
> diff --git a/remote-curl.c b/remote-curl.c
> index 762a55a75f..c89fd6d1c3 100644
> --- a/remote-curl.c
> +++ b/remote-curl.c
> @@ -618,9 +618,10 @@ static int probe_rpc(struct rpc_state *rpc, struct
> slot_results *results)
> }
>
> static curl_off_t xcurl_off_t(ssize_t len) {
> - if (len > maximum_signed_value_of_type(curl_off_t))
Is the issue that len is signed and maximum_signed_value_of_type()
gives an unsigned value, and these two are compared? As we saw
earlier, signed_add_overflows() is another example that wants a
mixed comparison.
I am just wondering if casting len to uintmax_t before comparing
with maximum_signed_value_of_type() is a simpler solution that can
safely be cargo-culted to other places without much thinking.
"git grep maximum_signed_value_of_type" reports a handful
comparisons in vcs-svn/, all of which does
if (var > maximum_signed_value_of_type(off_t))
with var of type uintmax_t, which sounds like a sane thing to do.
Thanks.
> + curl_off_t size = (curl_off_t) len;
> + if (len != (ssize_t) size)
> die("cannot handle pushes this big");
> - return (curl_off_t) len;
> + return size;
> }
[PATCH v1 2/2] curl_off_t xcurl_off_t is not portable
From: Torsten Bögershausen
Comparing signed and unsigned values is not always portable.
When setting
DEVELOPER = 1
DEVOPTS = extra-all
"gcc (Raspbian 6.3.0-18+rpi1+deb9u1) 6.3.0 20170516" errors out with
"comparison is always false due to limited range of data type"
"[-Werror=type-limits]"
Solution:
Use a valid cast & compare, similar to xsize_t()
Signed-off-by: Torsten Bögershausen
---
remote-curl.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/remote-curl.c b/remote-curl.c
index 762a55a75f..c89fd6d1c3 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -618,9 +618,10 @@ static int probe_rpc(struct rpc_state *rpc, struct
slot_results *results)
}
static curl_off_t xcurl_off_t(ssize_t len) {
- if (len > maximum_signed_value_of_type(curl_off_t))
+ curl_off_t size = (curl_off_t) len;
+ if (len != (ssize_t) size)
die("cannot handle pushes this big");
- return (curl_off_t) len;
+ return size;
}
static int post_rpc(struct rpc_state *rpc)
--
2.11.0
