linux-kernel - Re: [PATCH] fixdep: handle short reads in read

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Message-ID: <87y143ixdb.fsf@gentoo.org>
Date: Sat, 07 Sep 2024 11:14:40 +0100
From: Sam James <sam@...too.org>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>,  Nicolas Schier
 <nicolas@...sle.eu>,  linux-kbuild@...r.kernel.org,
  linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fixdep: handle short reads in read_file

Masahiro Yamada <masahiroy@...nel.org> writes:

> On Sat, Sep 7, 2024 at 12:29 AM Sam James <sam@...too.org> wrote:

Hi Masahiro,

>>
>> 50% or so of kernel builds within our package manager fail for me with
>> 'fixdep: read: success' because read(), for some reason - possibly ptrace,
>> only read a short amount, not the full size.
>>
>> Unfortunately, this didn't trigger a -Wunused-result warning because
>> we _are_ checking the return value, but with a bad comparison (it's completely
>> fine for read() to not read the whole file in one gulp).
>>
>> Fixes: 01b5cbe7012fb1eeffc5c143865569835bcd405e
>
>
> Fixes: 01b5cbe7012f ("fixdep: use malloc() and read() to load dep_file
> to buffer")
>

Ah, thanks. I'll fix that and send v2 depending on how we decide to move
forward wrt below.

>
> I guess, another approach would be to use fread() instead of read().
>
> Does the attached diff fix the issue too?
>
>

Unfortunately no. It failed for me in the same way as before :(

The man page mentions:
> On  success, fread() and fwrite() return the number of items read or
> written. This number equals the number of bytes transferred only when size is 1.  

so I guess it suffers from the same pitfall. I checked POSIX & ISO C as well
which says:
> If a partial element is read, its value is unspecified.
and
> The fread() function shall return the number of elements successfully
> read, which shall be less than nitems only if an error or end-of-file
> is encountered, or size is zero.

The error reference is kind of mysterious there though.

It kind of looks like fread *should* work. I'll send this mail and then
think about it a bit later and ask around to see if I'm missing
something obvious?

> [...]

thanks,
sam