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Top Forums UNIX for Dummies Questions & Answers x86 Interrupts and system calls Post 302606467 by ab_tall on Sunday 11th of March 2012 03:56:47 PM
Old 03-11-2012
Hmm so as I understand it,

int 0x80 causes the change to the kernel mode.

here 0x80 is used to index the IDT to get the location of the ISR.

What does the ISR location usually contain?

Does the ISR have code to use the system call no. from EAX to look into the dispatch table?
Also, I guess i'll have to look into how the dispatch table is populated in the first place.

The confusing part for me is that there seem to be 2 jumps here once we are in kernel mode.
1) Look up Interrupt vector table and jump to ISR
2) Look up dispatch table and jump to System call handler.

Please correct me if I'm wrong.

This book is what I'm referring to: Amazon.com: Understanding the Linux Kernel, Third Edition (9780596005658): Daniel P. Bovet, Marco Cesati Ph.D.: Books
 

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GET_KERNEL_SYMS(2)					     Linux Programmer's Manual						GET_KERNEL_SYMS(2)

NAME
get_kernel_syms - retrieve exported kernel and module symbols SYNOPSIS
#include <linux/module.h> int get_kernel_syms(struct kernel_sym *table); Note: No declaration of this system call is provided in glibc headers; see NOTES. DESCRIPTION
Note: This system call is present only in kernels before Linux 2.6. If table is NULL, get_kernel_syms() returns the number of symbols available for query. Otherwise, it fills in a table of structures: struct kernel_sym { unsigned long value; char name[60]; }; The symbols are interspersed with magic symbols of the form #module-name with the kernel having an empty name. The value associated with a symbol of this form is the address at which the module is loaded. The symbols exported from each module follow their magic module tag and the modules are returned in the reverse of the order in which they were loaded. RETURN VALUE
On success, returns the number of symbols copied to table. On error, -1 is returned and errno is set appropriately. ERRORS
There is only one possible error return: ENOSYS get_kernel_syms() is not supported in this version of the kernel. VERSIONS
This system call is present on Linux only up until kernel 2.4; it was removed in Linux 2.6. CONFORMING TO
get_kernel_syms() is Linux-specific. NOTES
This obsolete system call is not supported by glibc. No declaration is provided in glibc headers, but, through a quirk of history, glibc versions before 2.23 did export an ABI for this system call. Therefore, in order to employ this system call, it was sufficient to manually declare the interface in your code; alternatively, you could invoke the system call using syscall(2). BUGS
There is no way to indicate the size of the buffer allocated for table. If symbols have been added to the kernel since the program queried for the symbol table size, memory will be corrupted. The length of exported symbol names is limited to 59 characters. Because of these limitations, this system call is deprecated in favor of query_module(2) (which is itself nowadays deprecated in favor of other interfaces described on its manual page). SEE ALSO
create_module(2), delete_module(2), init_module(2), query_module(2) COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/. Linux 2017-09-15 GET_KERNEL_SYMS(2)
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