Example System Software

[Laura N Gerard]

Assilly as it may sound, I wasn't able to carve a good example. Everything (accessible to second-year students) that came to my mind involved system call in some way, and looking around for examples didn't help.

Both of these statements have exceptions (I/O through memory-mapped files or peripherals is I/O without syscalls, calling a hardware accelerator or allocating memory is syscalls without externally visible I/O) but they're a good approximation.

It would help to demonstrate by showing the system calls made by some programs, e.g. with strace on Linux, with dtruss on macOS or with Systrace under Windows. Compare a program that does a lot of I/O with one that does one big computation and prints the result.

There is sometimes possible confusion between a library call and a system call. Unix programmers in particular tend to believe that what is documented in section 2 of the manual is system calls, but this is not true on a modern Unix systems. Every function callable from C is a C library function that's a wrapper (possibly very simple, possibly even a macro rather than a function) around a system call that usually, but not always, has the same name and takes more or less the same parameters. You can observe that on a modern Linux/x86_32 system, where many system calls have been upgraded from 32-bit arguments to 64-bit arguments, e.g. strace ls will show calls of getdents64 and fstat64 rather than getdents and fstat. Under the hood, system calls and library calls have a different calling convention. At the very least, a library call involes a jump/branch instruction while a system call involves a privilege change instruction. Depending on the system, the rules for placing arguments in registers may be different.

You can discuss how a program could make as few system calls as possible by starting from strace /bin/true. Most of its syscalls are due to dynamic linking, so write that one-line program, compile it statically and look at the remaining system calls. Reduce it even further with a more minimalistic libc such as dietlibc.

"sys_call_table" is no longer exported in 2.6, so I'm just grabbing the address from the System.map file, and I can see it is correct (Looking through the memory at the address I found, I can see the pointers to the system calls).

This is CentOS 5.4 running 2.6.18-164.10.1.el5. Is there some sort of protection or do I just have a bug? I know it comes with SELinux, and I've tried putting it in to permissive mode, but it doesn't make a difference

I found this out after debugging why I still got "unable to handle kernel paging request" when trying to modify the address of kernel functions. I was eventually able to solve that problem by finding the page table entry for the address myself and manually setting it to writable. Thankfully, the lookup_address() function is exported in version 2.6.26+. Here is the code I wrote to do that:

As others have hinted, the whole story is a bit different now on modern kernels. I'll be covering x86-64 here, for syscall hijacking on modern arm64 refer to this other answer of mine.

Additionally, as others have described already in different answers, the simplest way to modify sys_call_table is to temporarily disable CR0 WP (Write-Protect) bit, which could be done using read_cr0() and write_cr0(). However, since Linux v5.3, [native_]write_cr0 will check sensitive bits that should never change (like WP) and refuse to change them (commit). In order to work around this, we need to write CR0 manually using inline assembly.

Here is a working kernel module (tested on Linux 5.10 and 5.18) that does syscall hijacking on modern Linux x86-64 considering the above caveats and assuming that you already know the address of sys_call_table (if you also want to find that in the module, see Proper way of getting the address of non-exported kernel symbols in a Linux kernel module):

One way I have been able to work around this is, provide datetime in a different format on the system prompt. add restrictions within the function to restrict it to ISO format. if not in ISO then throw an exception. this way now the model, calls the function with the datetime in the prompt, but as soon as it sees the exception, it calls get_current_datetime() and then retries the get_events function.

Well, the ChatGPT model is tuned to expect the time and date in the system prompt, so that sort of makes sense if that is also tuned into the GPT-3.5 and 4 models, one thing to also try is to raise the temperature a little try it at 0.5-0.9 to give it some explorative ability.

In this section, you can find example file system policies that grant or deny permissions for various Amazon EFS actions. Amazon EFS file system policies have a 20,000 character limit. For information about the elements of a resource-based policy, see Resource-based policies within Amazon EFS.

If you grant permission to an individual IAM user or role in a file system policy, don't delete or recreate that user or role while the policy is in effect on the file system. If this happens, that user or role is effectively locked out from file system and will not be able to access it. For more information, see Specifying a Principal in the IAM User Guide.

To learn how to set additional file system policies, including denying root access to all IAM principals, except for a specific management workstation, see Enable root squashing using IAM authorization for NFS clients.

You use an EFS access policy to provide an NFS client with an application-specific view into shared file-based datasets on an EFS file system. You grant the access point permissions on the file system using a file system policy.

The above example creates three equal-width columns on small, medium, large, and extra large devices using our predefined grid classes. Those columns are centered in the page with the parent .container.

While Bootstrap uses ems or rems for defining most sizes, pxs are used for grid breakpoints and container widths. This is because the viewport width is in pixels and does not change with the font size.

For example, here are two grid layouts that apply to every device and viewport, from xs to xl. Add any number of unit-less classes for each breakpoint you need and every column will be the same width.

Auto-layout for flexbox grid columns also means you can set the width of one column and have the sibling columns automatically resize around it. You may use predefined grid classes (as shown below), grid mixins, or inline widths. Note that the other columns will resize no matter the width of the center column.

Create equal-width columns that span multiple rows by inserting a .w-100 where you want the columns to break to a new line. Make the breaks responsive by mixing the .w-100 with some responsive display utilities.

For grids that are the same from the smallest of devices to the largest, use the .col and .col-* classes. Specify a numbered class when you need a particularly sized column; otherwise, feel free to stick to .col.

Breaking columns to a new line in flexbox requires a small hack: add an element with width: 100% wherever you want to wrap your columns to a new line. Normally this is accomplished with multiple .rows, but not every implementation method can account for this.

Use .order- classes for controlling the visual order of your content. These classes are responsive, so you can set the order by breakpoint (e.g., .order-1.order-md-2). Includes support for 1 through 12 across all five grid tiers.

You can offset grid columns in two ways: our responsive .offset- grid classes and our margin utilities. Grid classes are sized to match columns while margins are more useful for quick layouts where the width of the offset is variable.

To nest your content with the default grid, add a new .row and set of .col-sm-* columns within an existing .col-sm-* column. Nested rows should include a set of columns that add up to 12 or fewer (it is not required that you use all 12 available columns).

Variables and maps determine the number of columns, the gutter width, and the media query point at which to begin floating columns. We use these to generate the predefined grid classes documented above, as well as for the custom mixins listed below.

The number of grid columns can be modified via Sass variables. $grid-columns is used to generate the widths (in percent) of each individual column while $grid-gutter-width allows breakpoint-specific widths that are divided evenly across padding-left and padding-right for the column gutters.

Want to collaborate on code errors? Have bugs you need feedback on? Looking for an extra set of eyes on your latest project? Get support with fellow developers, designers, and programmers of all backgrounds and skill levels here with the Treehouse Community! While you're at it, check out some resources Treehouse students have shared here.

printf is used when you want to format your string. This will clean up any concatenation. For a simple example -> ("Hello, " + username + "! How are you?") could easily be cleaned up a bit by using ("Hello, %s! How are you?", username). It's up to you really when you want to use this.

I would definitely recommend it if you are using multiple variables. In that case you would use multiple conversions in your string. Please feel free to read up on the Formatter Documentation as it will help you understand it better and it provides a full list of available conversion characters.

The println() way of doing things would be to hard code spaces and tabs and run things a hundred times til everything lined up, or writing your own functions to format strings before plugging the string into the println method.

println() prints a new blank line and then your message.printf() provides string formatting similar to the printf function in C. printf() is primarily needed when you need to print big strings to avoid string concatenaion in println() which can be confusing at times.(Although both can be used in almost all cases).

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