Understanding of asmlinkage

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One 、 In large scale C Language project engineering or linux We often see two in the kernel FASTCALL and armlinkage

      Two identifiers ( Modifier ), So what are the differences between them ？ Today I will share my experience with you .

      Everyone knows that in the standard C The formal parameters of functions in the system will involve the problem of parameter storage when actually passing in parameters , Where are these parameters stored ？ Friends with a certain theoretical basis will definitely answer ： These function parameters and local variables inside the function are allocated to the local stack of the function , Is that really the case ？ In fact, there are exceptions ：

      First as linux operating system , It doesn't necessarily only run on X86 Under the platform , There are other platforms such as ARM,PPC, Da Vinci, wait , Therefore, different processor architectures cannot be guaranteed to pass The local stack passes parameters , Maybe a friend will ask at this time ： Where can I put it if I don't put it on the stack ？ be familiar with ARM My friend must know ARM It defines a set of rules for parameters passing in the process of function call , It's called ATPCS（ The mainland is called AAPCS）, The rules clearly state ARM in R0-R4 Are used as general registers , When the function is called, the processor starts from R0-R4 Get parameters in , When the function returns Save the parameters to be returned to R0-R4 in , In other words, function parameters can be stored directly in registers , So in order to strictly distinguish the storage location of function parameters , Two tags are introduced , namely asmlinkage and FASTCALL, The former means that the function parameters are stored in the local stack , The latter tells the compiler to save the function parameters in registers

      We are searching for some additional clues ,ARM in R0-R4 Used to store incoming parameters , Vaguely tell us , As a high-level embedded system developer , Or high level C Language programmers , The parameters of a function should not This is greater than 5 individual , Then someone will ask in turn ： exceed 5 Where will those parameters of go ？ My answer is ： If the incoming parameter exceeds 5 individual , Redundant parameters are still stored in the local stack , At this time, someone may May ask ： What's wrong with passing function parameters into the local stack ？ My answer is ： There is nothing bad on the surface , But if you are a person with linux Kernel trained programmers , You will vaguely remember linux in , Whether it's a system call , Or system traps will cause user space to fall into kernel space , We know , The permission level of system space is 0, The permission level of user space is 3, The system call level is 3 User space trap The permission level is 0 The kernel space of , It will inevitably cause stack switching ,linux The system will start from the global task status stack TSS Find a suitable kernel stack information to save and overwrite the current SP,SS The inside of two registers Rong , To complete stack switching , At this time, the stack seen in kernel space is not the stack in user space , So the data pushed into the user stack at the time of call is at the moment when it gets into the kernel , Stranded in the user space stack , The kernel doesn't even know it exists , Therefore, as a security consideration or as a personal cultivation of high-level programmers , You should not pass too many parameters to system call level functions .

  Two ： It is GCC Yes C An extension of the program , #define asmlinkage __attribute__((regparm(0)))

      To express with 0 Register transfer function parameters , such , All function parameters are forced to be extracted from the stack .
This asmlinkage Mostly used in system calls , The system call needs to be in entry.s In the file, assembly language is used to call , So we must ensure that it conforms to C Parameter passing rules of language , It can be called correctly in assembly language .
That's why asmlinkage Why? ！ This is my understanding .

Take a closer look, there is asmlinkage The place where is usually the function called by the system , Because in the system call , After the register is transferred from user space SAVE_ALL Push into the stack , Then call the corresponding system call function , In this way, the system call function must ensure that the parameters are passed through the stack
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