linux-kernel - Re: [PATCH] fs/9p: Fix a datatype used with V9FS_DIRECT

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Date:   Tue, 25 Apr 2023 19:40:22 +0900
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Christian Schoenebeck <linux_oss@...debyte.com>
Cc:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Eric Van Hensbergen <ericvh@...nel.org>,
        Latchesar Ionkov <lucho@...kov.net>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        v9fs@...ts.linux.dev
Subject: Re: [PATCH] fs/9p: Fix a datatype used with V9FS_DIRECT_IO

Christian Schoenebeck wrote on Tue, Apr 25, 2023 at 11:18:37AM +0200:
> > I'm surprised W=1 doesn't catch this... and now I'm checking higher
> > (noisy) W=, or even clang doesn't seem to print anything about e.g.
> > 'v9ses->flags & V9FS_DIRECT_IO is never true' or other warnings I'd have
> > expected to come up -- out of curiosity how did you find this?
> 
> Both gcc and clang only trigger an implicit conversion warning if the value of
> the expression can be evaluated at compile time (i.e. all operands are
> constant), then compiler realizes that the compile-time evaluated constant
> value is too big for the assignment destination and triggers the warning.

Right, `v9ses->flags = V9FS_DIRECT_IO` would have triggered it but not
with `|=` -- but in this case I was also expecting the check
`v9ses->flags & V9fs_DIRECT_IO` to flag something odd...
But nothing seems to care; testing with this snippet:
---
int foo(char x) {
	if (x & 0x200)
		return 1;
	return 0;
}
int foo2(unsigned char x) {
	if (x < 0)
		return 1;
	return 0;
}
---
gcc warns that the x < 0 is always false (clang actually doesn't, even
with scan-build, I must be missing a flag?), but I didn't find anything
complaining about the &.
I'd expect something like coverity to perform a bit better here but it's
a pain to use the "free for open source" version (... I just requested
access to https://scan.coverity.com/projects/128 but I have no idea if
they build next or not)

Oh, well; glad Christophe noticed anyway.

> > Would probably be interesting to run some form of the same in our
> > automation.
> 
> If there is any ATM? I als tried this issue with clang's undefined behaviour
> sanitizer and with the clang static analyzer. Both did not detect it.

There's at least the intel bot building with W=1 and warning if any new
such warning pops up (and I'd like to say I check myself, but I probably
forget about half the time; I looked at making W=1 default for our part
of the tree but it didn't look trivial? I'll try to have another look);
but I'm not aware of anyone testing with scan-build or something else
that'd contact us on new defects.

-- 
Dominique Martinet | Asmadeus