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From: "René Scharfe" <l.s.r@web.de>
To: Johannes Schindelin via GitGitGadget <gitgitgadget@gmail.com>,
	git@vger.kernel.org
Cc: Junio C Hamano <gitster@pobox.com>,
	Johannes Schindelin <johannes.schindelin@gmx.de>
Subject: Re: [PATCH 4/4] config: avoid calling `labs()` on too-large data type
Date: Sun, 16 Jun 2019 10:24:17 +0200	[thread overview]
Message-ID: <f22599d4-148f-212c-282c-790f5bde1706@web.de> (raw)
In-Reply-To: <03a68560-8058-7436-6edb-38e276a37190@web.de>

Am 16.06.19 um 08:48 schrieb René Scharfe:
> Am 13.06.19 um 13:49 schrieb Johannes Schindelin via GitGitGadget:
>> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>>
>> The `labs()` function operates, as the initial `l` suggests, on `long`
>> parameters. However, in `config.c` we tried to use it on values of type
>> `intmax_t`.
>>
>> This problem was found by GCC v9.x.
>>
>> To fix it, let's just "unroll" the function (i.e. negate the value if it
>> is negative).
>
> There's also imaxabs(3).
>
>>
>> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
>> ---
>>  config.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/config.c b/config.c
>> index 296a6d9cc4..01c6e9df23 100644
>> --- a/config.c
>> +++ b/config.c
>> @@ -869,9 +869,9 @@ static int git_parse_signed(const char *value, intmax_t *ret, intmax_t max)
>>  			errno = EINVAL;
>>  			return 0;
>>  		}
>> -		uval = labs(val);
>> +		uval = val < 0 ? -val : val;
>>  		uval *= factor;
>> -		if (uval > max || labs(val) > uval) {
>> +		if (uval > max || (val < 0 ? -val : val) > uval) {
>>  			errno = ERANGE;
>>  			return 0;
>>  		}
>
> So this check uses unsigned arithmetic to find out if the multiplication
> overflows, right?  Let's say value is "4G", then val will be 4 and
> factor will be 2^30.  Multiplying the two yields 2^32.  On a 32-bit
> system this will wrap around to 0, so that's what we get for uval there.
> The range check will then pass unless max is negative, which is wrong.

No, this example is wrong.  (I need to remember to take baby steps while
carrying numbers. o_O)

So value = "5G", then val = 5 and factor = 2^30.  After multiplication
uval = 2^32 + 2^30, on 32-bit machines this is 2^30 due to wrap-around.
Correct so far?

If uval is 2^30, then it's smaller than 2^31, so will pass a check
against INT_MAX.  val is 5, which is smaller than 2^30, so will pass the
second check as well.  Makes sense?

That would mean "5G" will overflow on a 32-bit machine, but we won't
detect it.

René

  reply	other threads:[~2019-06-16  8:24 UTC|newest]

Thread overview: 90+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-13 11:49 [PATCH 0/4] Support building with GCC v8.x/v9.x Johannes Schindelin via GitGitGadget
2019-06-13 11:49 ` [PATCH 1/4] poll (mingw): allow compiling with GCC 8 and DEVELOPER=1 Johannes Schindelin via GitGitGadget
2019-06-13 11:49 ` [PATCH 2/4] kwset: allow building with GCC 8 Johannes Schindelin via GitGitGadget
2019-06-13 16:11   ` Junio C Hamano
2019-06-14  9:53   ` SZEDER Gábor
2019-06-14 10:00     ` [RFC/PATCH v1 0/4] compat/obstack: update from upstream SZEDER Gábor
2019-06-14 10:00       ` [PATCH v1 1/4] " SZEDER Gábor
2019-06-14 10:00       ` [PATCH v1 2/4] SQUASH??? compat/obstack: fix portability issues SZEDER Gábor
2019-06-14 10:00       ` [PATCH v1 3/4] SQUASH??? compat/obstack: fix build errors with Clang SZEDER Gábor
2019-06-14 10:00       ` [PATCH v1 4/4] compat/obstack: fix some sparse warnings SZEDER Gábor
2019-06-14 17:57       ` [RFC/PATCH v1 0/4] compat/obstack: update from upstream Jeff King
2019-06-14 18:19       ` Junio C Hamano
2019-06-14 20:30       ` Ramsay Jones
2019-06-14 21:24         ` Ramsay Jones
2019-06-17 18:36         ` SZEDER Gábor
2019-06-14 16:12     ` [PATCH 2/4] kwset: allow building with GCC 8 Junio C Hamano
2019-06-17 18:26       ` SZEDER Gábor
2019-06-14 22:09   ` Ævar Arnfjörð Bjarmason
2019-06-14 22:55   ` Can we just get rid of kwset & obstack in favor of optimistically using PCRE v2 JIT? Ævar Arnfjörð Bjarmason
2019-06-14 23:19     ` Ævar Arnfjörð Bjarmason
2019-06-20 10:35       ` Jeff King
2019-06-15  9:01     ` Carlo Arenas
2019-06-15 19:15     ` brian m. carlson
2019-06-15 22:14       ` Ævar Arnfjörð Bjarmason
2019-06-26  0:03         ` [RFC/PATCH 0/7] grep: move from kwset to optional PCRE v2 Ævar Arnfjörð Bjarmason
2019-06-26 14:02           ` Johannes Schindelin
2019-06-27  9:16             ` Johannes Schindelin
2019-06-27 16:27               ` Ævar Arnfjörð Bjarmason
2019-06-27 18:21                 ` Johannes Schindelin
2019-06-27 23:39           ` [PATCH v2 0/9] " Ævar Arnfjörð Bjarmason
2019-06-28  7:23             ` Ævar Arnfjörð Bjarmason
2019-06-28 16:10               ` Junio C Hamano
2019-07-01 21:20             ` [PATCH v3 00/10] " Ævar Arnfjörð Bjarmason
2019-07-01 21:31               ` Junio C Hamano
2019-07-02 11:10                 ` Ævar Arnfjörð Bjarmason
2019-07-02 12:32               ` Johannes Schindelin
2019-07-02 19:57                 ` Junio C Hamano
2019-07-03 10:08                   ` Johannes Schindelin
2019-07-03 10:25                 ` Johannes Schindelin
2019-07-03 11:27                   ` Johannes Schindelin
2019-07-01 21:20             ` [PATCH v3 01/10] log tests: test regex backends in "--encode=<enc>" tests Ævar Arnfjörð Bjarmason
2019-07-01 21:20             ` [PATCH v3 02/10] grep: don't use PCRE2?_UTF8 with "log --encoding=<non-utf8>" Ævar Arnfjörð Bjarmason
2019-07-01 21:20             ` [PATCH v3 03/10] t4210: skip more command-line encoding tests on MinGW Ævar Arnfjörð Bjarmason
2019-07-01 21:20             ` [PATCH v3 04/10] grep: inline the return value of a function call used only once Ævar Arnfjörð Bjarmason
2019-07-01 21:20             ` [PATCH v3 05/10] grep tests: move "grep binary" alongside the rest Ævar Arnfjörð Bjarmason
2019-07-01 21:20             ` [PATCH v3 06/10] grep tests: move binary pattern tests into their own file Ævar Arnfjörð Bjarmason
2019-07-01 21:20             ` [PATCH v3 07/10] grep: make the behavior for NUL-byte in patterns sane Ævar Arnfjörð Bjarmason
2019-07-01 21:20             ` [PATCH v3 08/10] grep: drop support for \0 in --fixed-strings <pattern> Ævar Arnfjörð Bjarmason
2019-07-01 21:20             ` [PATCH v3 09/10] grep: remove the kwset optimization Ævar Arnfjörð Bjarmason
2019-07-01 21:21             ` [PATCH v3 10/10] grep: use PCRE v2 for optimized fixed-string search Ævar Arnfjörð Bjarmason
2019-06-27 23:39           ` [PATCH v2 1/9] log tests: test regex backends in "--encode=<enc>" tests Ævar Arnfjörð Bjarmason
2019-06-27 23:39           ` [PATCH v2 2/9] grep: don't use PCRE2?_UTF8 with "log --encoding=<non-utf8>" Ævar Arnfjörð Bjarmason
2019-06-27 23:39           ` [PATCH v2 3/9] grep: inline the return value of a function call used only once Ævar Arnfjörð Bjarmason
2019-06-27 23:39           ` [PATCH v2 4/9] grep tests: move "grep binary" alongside the rest Ævar Arnfjörð Bjarmason
2019-06-27 23:39           ` [PATCH v2 5/9] grep tests: move binary pattern tests into their own file Ævar Arnfjörð Bjarmason
2019-06-27 23:39           ` [PATCH v2 6/9] grep: make the behavior for NUL-byte in patterns sane Ævar Arnfjörð Bjarmason
2019-06-27 23:39           ` [PATCH v2 7/9] grep: drop support for \0 in --fixed-strings <pattern> Ævar Arnfjörð Bjarmason
2019-06-27 23:39           ` [PATCH v2 8/9] grep: remove the kwset optimization Ævar Arnfjörð Bjarmason
2019-06-27 23:39           ` [PATCH v2 9/9] grep: use PCRE v2 for optimized fixed-string search Ævar Arnfjörð Bjarmason
2019-06-26  0:03         ` [RFC/PATCH 1/7] grep: inline the return value of a function call used only once Ævar Arnfjörð Bjarmason
2019-06-26  0:03         ` [RFC/PATCH 2/7] grep tests: move "grep binary" alongside the rest Ævar Arnfjörð Bjarmason
2019-06-26 14:05           ` Johannes Schindelin
2019-06-26 18:13           ` Junio C Hamano
2019-06-26  0:03         ` [RFC/PATCH 3/7] grep tests: move binary pattern tests into their own file Ævar Arnfjörð Bjarmason
2019-06-26  0:03         ` [RFC/PATCH 4/7] grep: make the behavior for \0 in patterns sane Ævar Arnfjörð Bjarmason
2019-06-27  2:03           ` brian m. carlson
2019-06-26  0:03         ` [RFC/PATCH 5/7] grep: drop support for \0 in --fixed-strings <pattern> Ævar Arnfjörð Bjarmason
2019-06-26 16:14           ` Junio C Hamano
2019-06-26  0:03         ` [RFC/PATCH 6/7] grep: remove the kwset optimization Ævar Arnfjörð Bjarmason
2019-06-26  0:03         ` [RFC/PATCH 7/7] grep: use PCRE v2 for optimized fixed-string search Ævar Arnfjörð Bjarmason
2019-06-26 14:13           ` Johannes Schindelin
2019-06-26 18:45             ` Junio C Hamano
2019-06-27  9:31               ` Johannes Schindelin
2019-06-27 18:45                 ` Johannes Schindelin
2019-06-27 19:06                   ` Junio C Hamano
2019-06-28 10:56                     ` Johannes Schindelin
2019-06-13 11:49 ` [PATCH 3/4] winansi: simplify loading the GetCurrentConsoleFontEx() function Johannes Schindelin via GitGitGadget
2019-06-13 11:49 ` [PATCH 4/4] config: avoid calling `labs()` on too-large data type Johannes Schindelin via GitGitGadget
2019-06-13 16:13   ` Junio C Hamano
2019-06-16  6:48   ` René Scharfe
2019-06-16  8:24     ` René Scharfe [this message]
2019-06-16 14:01       ` René Scharfe
2019-06-16 22:26         ` Junio C Hamano
2019-06-20 19:58           ` René Scharfe
2019-06-20 21:07             ` Junio C Hamano
2019-06-21 18:35             ` Johannes Schindelin
2019-06-22 10:03               ` René Scharfe
2019-06-22 10:03           ` [PATCH v2 1/3] config: use unsigned_mult_overflows to check for overflows René Scharfe
2019-06-22 10:03           ` [PATCH v2 2/3] config: don't multiply in parse_unit_factor() René Scharfe
2019-06-22 10:03           ` [PATCH v2 3/3] config: simplify parsing of unit factors René Scharfe

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