Question Summary:
My question is that is it ok to perform salat(Namaz) in shorts if the length cover the knees.

Question Detail:

My question is that is it ok to perform salat(Namaz) in shorts if the length cover the knees.

Answer :

In the name of Allah, Most Gracious, Most Merciful
Assalaamu `alaykum waRahmatullahi Wabarakatuh
In order for salaah to be valid the satr must be covered. The satr for a male is from the navel till below the knees. This is clearly substantiated in a hadeeth: 
قال النبي صلي الله عليه وسلم :عورة الرجل ما بين سرةه الا ركبته .اخرجه الدار قطني.
(Al Hidaayah V3-4 Pg 460 Shirkat e Ilmiyah)
The shorts covering the knees will fulfil this minimum requirement. However salaah is a means of communication with our creator.  One should adorn such attire that invokes the mercy of Allah Taala.To adorn oneself with the sunnah attire is a mark of respect for Allah Taala.
Therefore to suffice on the shorts is undesirable.
And Allah knows best
Wassalam
Ml. Ebrahim Desai,
Student Darul Iftaa
Checked and Approved by:
Mufti Ebrahim Desai
Darul Iftaa, Madrassah In'aamiyyah