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Re: [Qemu-devel] [PATCH] check for available room when formatting OpenFi
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH] check for available room when formatting OpenFirmware device path |
Date: |
Mon, 23 Jul 2012 17:42:15 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.0.97 (gnu/linux) |
Laszlo Ersek <address@hidden> writes:
> On 07/23/12 17:01, Markus Armbruster wrote:
>> Laszlo Ersek <address@hidden> writes:
>>
>>> On 07/23/12 14:46, Markus Armbruster wrote:
>>>> Laszlo Ersek <address@hidden> writes:
>>>>
>>>>> Signed-off-by: Laszlo Ersek <address@hidden>
>>>>> ---
>>>>> hw/qdev.c | 14 +++++++++++++-
>>>>> vl.c | 7 ++++++-
>>>>> 2 files changed, 19 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/hw/qdev.c b/hw/qdev.c
>>>>> index af54467..f1e83a4 100644
>>>>> --- a/hw/qdev.c
>>>>> +++ b/hw/qdev.c
>>>>> @@ -502,6 +502,10 @@ static int qdev_get_fw_dev_path_helper(DeviceState
>>>>> *dev, char *p, int size)
>>>>> if (dev && dev->parent_bus) {
>>>>> char *d;
>>>>> l = qdev_get_fw_dev_path_helper(dev->parent_bus->parent, p,
>>>>> size);
>>>>> + if (l >= size) {
>>>>> + return l;
>>>>> + }
>>>>> +
>>>>> d = bus_get_fw_dev_path(dev->parent_bus, dev);
>>>>> if (d) {
>>>>> l += snprintf(p + l, size - l, "%s", d);
>>>>> @@ -509,6 +513,10 @@ static int qdev_get_fw_dev_path_helper(DeviceState
>>>>> *dev, char *p, int size)
>>>>> } else {
>>>>> l += snprintf(p + l, size - l, "%s",
>>>>> object_get_typename(OBJECT(dev)));
>>>>> }
>>>>> +
>>>>> + if (l >= size) {
>>>>> + return l;
>>>>> + }
>>>>> }
>>>>> l += snprintf(p + l , size - l, "/");
>>>>>
>>>>
>>>> If the return value is less than the size argument, it's the length of
>>>> the string written into p[]. Else, it means p[] has insufficient
>>>> space.
>>>
>>> Yes. (snprintf() returns the number of bytes it would store, excluding
>>> the terminating NUL, had there been enough room.
>>> <http://pubs.opengroup.org/onlinepubs/9699919799/functions/snprintf.html#tag_16_159_04>)
>>>
>>> Did I make a mistake?
>>>
>>> Supposing snprintf() encounters no error, it returns a positive value P
>>> in the above.
>>>
>>> P = snprintf(..., size - l0, ...)
>>> l1 = l0 + P;
>>>
>>> l1 >= size
>>> <-> l0 + P >= size
>>> <-> P >= size - l0
>>>
>>>
>>> The return value of qdev_get_fw_dev_path_helper() comes from another
>>> invocation of itself, or from the trailing snprintf(), so it behaves
>>> like snprintf.
>>>
>>> Or what do you have in mind?
>>
>> If I read your code correctly, qdev_get_fw_dev_path_helper() bails out
>> after the first snprintf() that goes beyond the buffer size. When that
>> happens before the job's done, the return value is less than the length
>> of the full path.
>
> Yes, but still greater than what would fit in the buffer. Is it a problem?
>
> ... Oh, did you mean the first comment in connection with the second?
> Ie. we should continue to format (just for length counting's sake) and
> then retry? IOW the early return isn't problematic in itself, but it
> doesn't support the reallocation?
Yes.
Your code lets the caller detect "need more space" reliably. How much
more it can only guess, and need to be prepared for the retry to fail
again. If we return the required size, like snprintf() does, the caller
knows, and the retry will succeed.
- [Qemu-devel] [PATCH] check for available room when formatting OpenFirmware device path, Laszlo Ersek, 2012/07/23
- Re: [Qemu-devel] [PATCH] check for available room when formatting OpenFirmware device path, Peter Maydell, 2012/07/23
- Re: [Qemu-devel] [PATCH] check for available room when formatting OpenFirmware device path, Markus Armbruster, 2012/07/23
- Re: [Qemu-devel] [PATCH] check for available room when formatting OpenFirmware device path, Laszlo Ersek, 2012/07/23
- Re: [Qemu-devel] [PATCH] check for available room when formatting OpenFirmware device path, Markus Armbruster, 2012/07/23
- Re: [Qemu-devel] [PATCH] check for available room when formatting OpenFirmware device path, Laszlo Ersek, 2012/07/23
- Re: [Qemu-devel] [PATCH] check for available room when formatting OpenFirmware device path,
Markus Armbruster <=
- [Qemu-devel] [PATCH v2 0/2] assorted device path formatting cleanups, Laszlo Ersek, 2012/07/24
- [Qemu-devel] [PATCH v2 1/2] accomodate OpenFirmware device paths in sufficient storage, Laszlo Ersek, 2012/07/24
- Re: [Qemu-devel] [PATCH v2 1/2] accomodate OpenFirmware device paths in sufficient storage, Peter Maydell, 2012/07/25
- Re: [Qemu-devel] [PATCH v2 1/2] accomodate OpenFirmware device paths in sufficient storage, Markus Armbruster, 2012/07/25
- [Qemu-devel] [PATCH v2 2/2] get_fw_dev_path() impls should allocate memory with glib functions, Laszlo Ersek, 2012/07/24