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From:   Junio C Hamano <gitster@pobox.com>
To:     Jonathan Tan <jonathantanmy@google.com>
Cc:     Marc Strapetz <marc.strapetz@syntevo.com>, git@vger.kernel.org
Subject: Re: [PATCH] packfile: avoid overflowing shift during decode
References: <20211110234033.3144165-1-jonathantanmy@google.com>
        <xmqqpmr7v4gf.fsf@gitster.g>
        <5ab9257c-9eba-a171-86d6-3fe7d3a4faec@syntevo.com>
Date:   Wed, 12 Jan 2022 12:06:01 -0800
In-Reply-To: <5ab9257c-9eba-a171-86d6-3fe7d3a4faec@syntevo.com> (Marc
        Strapetz's message of "Tue, 11 Jan 2022 00:22:04 +0100")
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Marc Strapetz <marc.strapetz@syntevo.com> writes:

> On 11/11/2021 02:58, Junio C Hamano wrote:
>> Jonathan Tan <jonathantanmy@google.com> writes:
>> 
>>> diff --git a/packfile.c b/packfile.c
>>> index 89402cfc69..972c327e29 100644
>>> --- a/packfile.c
>>> +++ b/packfile.c
>>> @@ -1068,7 +1068,7 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,
>>>   	size = c & 15;
>>>   	shift = 4;
>>>   	while (c & 0x80) {
>>> -		if (len <= used || bitsizeof(long) <= shift) {
>>> +		if (len <= used || (bitsizeof(long) - 7) <= shift) {
>
> This seems to cause troubles now for 32-bit systems (in my case Git
> for Windows 32-Bit): `shift` will go through 4, 11, 18 and for 25 it
> finally errors out. This means that objects >= 32MB can't be processed
> anymore. The condition should probably be changed to:
>
> +		if (len <= used || (bitsizeof(long) - 7) < shift) {
>
> This still ensures that the shift can never overflow and on 32-bit
> systems restores the maximum size of 4G with a final shift of 127<<25 
> (the old condition `bitsizeof(long) <= shift` was perfectly valid for
> 32-bit systems).

Jonathan?