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Re: [PATCH v8 6/7] xen/serial: harden serial_tx_buffer checks
- To: Stefano Stabellini <sstabellini@xxxxxxxxxx>, dmukhin@xxxxxxxx
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 12 Aug 2026 10:13:51 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, andrew.cooper3@xxxxxxxxxx, anthony.perard@xxxxxxxxxx, julien@xxxxxxx, michal.orzel@xxxxxxx, Roger Pau Monné <roger@xxxxxxxxxxxxxx>
- Delivery-date: Wed, 12 Aug 2026 08:14:02 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 10.08.2026 22:32, Stefano Stabellini wrote:
> On Mon, 27 Jul 2026, dmukhin@xxxxxxxx wrote:
>> --- a/xen/drivers/char/serial.c
>> +++ b/xen/drivers/char/serial.c
>> @@ -523,6 +523,8 @@ void __init serial_async_transmit(struct serial_port
>> *port)
>> return;
>> if ( serial_txbufsz < PAGE_SIZE )
>> serial_txbufsz = PAGE_SIZE;
>> + if ( serial_txbufsz > GB(2) )
>> + serial_txbufsz = CONFIG_SERIAL_TX_BUFSIZE;
>> while ( serial_txbufsz & (serial_txbufsz - 1) )
>> serial_txbufsz &= serial_txbufsz - 1;
>
> My understanding of this loop is that, given that serial_txbufsz is
> unsigned int, it is already clamping it to 2GB max
It would, yes, as long as sizeof(unsigned int) == sizeof(uint32_t). While
we certainly have many instances of that assumption in the code, I think
we're well advised to avoid adding more.
Furthermore (see related comments on patch 5) one may raise the question
whether serial_txbufsz wouldn't better be of type size_t.
What I'm uncertain about is the resetting to CONFIG_SERIAL_TX_BUFSIZE
here: When an overly huge size was requested, shouldn't we give the
biggest we permit?
Jan
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