Обсуждение: Detect buffer underflow in get_th()

Hello everyone,

In src/backend/utils/adt/formatting.c:1516, there is a get_th() function utilized to return ST/ND/RD/TH suffixes for
simplenumbers.
 
Upon reviewing its behavior, it appears capable of receiving non-numeric inputs (this is verified by a check at
formatting.c:1527).

Given that the function can accept non-numeric inputs,
it is plausible that it could also receive an empty input,
although a brief examination of its calls did not reveal any such instances.

Nevertheless, if the function were to receive an empty input of zero length,
a buffer underflow would occur when attempting to compute *(num + (len - 1)), as (len - 1) would result in a negative
shift.
To mitigate this issue, I propose a patch incorporating the zero_length_character_string error code, as detailed in the
attachment.

-- 
Best regards,
Alexander Kuznetsov

On 24.07.24 11:43, Alexander Kuznetsov wrote:
> Hello everyone,
> 
> In src/backend/utils/adt/formatting.c:1516, there is a get_th() function 
> utilized to return ST/ND/RD/TH suffixes for simple numbers.
> Upon reviewing its behavior, it appears capable of receiving non-numeric 
> inputs (this is verified by a check at formatting.c:1527).
> 
> Given that the function can accept non-numeric inputs,
> it is plausible that it could also receive an empty input,
> although a brief examination of its calls did not reveal any such 
> instances.
> 
> Nevertheless, if the function were to receive an empty input of zero 
> length,
> a buffer underflow would occur when attempting to compute *(num + (len - 
> 1)), as (len - 1) would result in a negative shift.
> To mitigate this issue, I propose a patch incorporating the 
> zero_length_character_string error code, as detailed in the attachment.

If it can't happen in practice, maybe an assertion would be enough?

24.07.2024 18:39, Peter Eisentraut wrote:
> If it can't happen in practice, maybe an assertion would be enough?
>

In practice, the function should not receive non-numeric strings either; however, since there is an exception in place
forsuch cases, I thought it would be good to add a check for zero-length input in a similar manner.
 

But of course it's open for discussion and team decision whether this should be addressed as an assertion or handled
differently.

-- 
Best regards,
Alexander Kuznetsov