Stephan Bridger

An insightful passage on dynamic linking, global offset tables, and procedure linkage tables, from Jan Hubicka : Much as the global offset table redirects position-independent address calculations to absolute locations, the procedure linkage table redirects position-independent function calls to absolute locations. The link editor cannot resolve execution transfers (such as function calls) from one executable or shared object to another. Consequently, the link editor arranges to have the program transfer control to entries in the procedure linkage table. On the AMD64 architecture, procedure linkage tables reside in shared text, but they use addresses in the private global offset table. The dynamic linker determines the destinations' absolute addresses and modifies the global offset table's memory image accordingly. The much lauded paper " How to Write Shared Libaries " by Ulrich Drepper explains this and much more in great detail. (ELf structures, relocations, sym...

Address space layout randomization, aka ASLR -- and position independent executables (PIE), are used to improve the security of modern operating systems by making memory addresses less predictable. Position-independent executables let systems more effectively use ASLR to randomize their memory layouts at runtime. The entry point offsets to functions remain fixed, while the base address is randomized. $ readelf -h /usr/bin/ls ELF Header: Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 Class: ELF64 Data: 2's complement, little endian Version: 1 (current) OS/ABI: UNIX - System V ABI Version: 0 Type: DYN (Position-Independent Executable file) Machine: Advanced Micro Devices X86-64 Version: 0x1 Entry point address: 0x6d30 Start ...

Implementing something that works but that you don't entirely understand is like finding a new chord that you don't know the name of yet. You have an intuition that it's a pleasant chord—it may work—but you might lack the ability to fully articulate or describe why. In a moment such as that, it's good to pause and consult the literature until you can fully articulate it. There's a subtle, incredible difference between implementing a thing and understanding a thing. If you find a way to do something but move forward without fully understanding why it works, the only thing you carry forward is a mistaken understanding about it. Its usefulness remains only a happy accident. Happy accidents are ok. They're sometimes useful if they're stepping stones on the way to greater learning. Practicing music is a lot like that. But repetition can be a double-edged sword. For example, if you make a mistake but don't stop and correct it—and instead just continue pl...

An interesting passage from G.K. Chesterton's "Orthodoxy" : All the will-worshippers, from Nietzsche to Mr. Davidson, are really quite empty of volition. They cannot will, they can hardly wish. And if any one wants a proof of this, it can be found quite easily. It can be found in this fact : that they always talk of will as something that expands and breaks out. But it is quite the opposite. Every act of will is an act of self-limitation. To desire action is to desire limitation. In that sense every act is an act of self-sacrifice. When you choose anything, you reject everything else. That objection, which men of this school used to make to the act of marriage, is really an objection to every act. Every act is an irrevocable selection and exclusion. Just as when you marry one woman you give up all the others, so when you take one course of action you give up all the other courses. If you become King of England, you give up the post of Beadle in Brompton. If you go to Ro...

Elfland Just as Windows has its various executable formats , so too does Linux. In this land, there are Elfs, also known as executable and linkable format files. If we look at elf.h , we can see the structures which constitute the ELF format : #define EI_NIDENT 16 typedef struct { unsigned char e_ident[EI_NIDENT]; Elf32_Half e_type; Elf32_Half e_machine; Elf32_Word e_version; Elf32_Addr e_entry; Elf32_Off e_phoff; Elf32_Off e_shoff; Elf32_Word e_flags; Elf32_Half e_ehsize; Elf32_Half e_phentsize; Elf32_Half e_phnum; Elf32_Half e_shentsize; Elf32_Half e_shnum; Elf32_Half e_shstrndx; } Elf32_Ehdr; typedef struct { unsigned char e_ident[EI_NIDENT]; Elf64_Half e_type; Elf64_Half e_machine; Elf64_Word e_version; Elf64_Addr e_entry; ...

For a little over a year, AppArmor has been broken on some distributions because of a two line bug that can be found here . On a default Ubuntu 24.04.1 LTS installation, trying to run aa-enforce /etc/apparmor.d/* to enable apparmor profiles fails with: Traceback (most recent call last): File "/usr/sbin/aa-enforce", line 33, in tool.cmd_enforce() File "/usr/lib/python3/dist-packages/apparmor/tools.py", line 134, in cmd_enforce for (program, prof_filename, output_name) in self.get_next_for_modechange(): File "/usr/lib/python3/dist-packages/apparmor/tools.py", line 97, in get_next_for_modechange aaui.UI_Info(_('Profile for %s not found, skipping') % output_name) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ TypeError: 'NoneType' object is not callable An unexpected error occurred! The bug has been fixed in AppArmor but the patch hasn't been pushed upstream to Ubuntu yet. If we pull down the raw corrected file, we can diff an...

This is a good post from Bruce Schneier's blog, originally published in 2011. While I can’t claim this list represents the top five cognitive biases in an empirical sense, it certainly covers a broad spectrum of the holes that tend to occur in human reasoning. We tend to exaggerate spectacular and rare risks and downplay common risks. The unknown is perceived to be riskier than the familiar. Personified risks are perceived to be riskier than anonymous risks. We underestimate risks in situations we do control, and overestimate risks in situations we don’t control. We estimate the probability of something by how easy it is to bring examples to mind. (cont.) ...Newspapers repeat rare risks again and again. When something is in the news, it is, by definition, something that almost never happens. Things that are so common they stop becoming newsworthy—like car accidents—are what you need to worry about. "The 5 Biggest Biases We Fall Victim to"

This morning I saw a post on twitter that said: The tragedy about waiting 6 months for something is that there aren't many "6 months" in a human life to wait. This was in the context of a discussion about various services and their relative speeds, e.g. Amazon delivery versus health care or city planning. Amazon might be able to deliver a thing tomorrow . Fixing a road, acquiring a city permit, or getting a doctor's appointment might take several months . But the general point is true in the long-view of life in its entirety -- the clock is always there, ticking. Time is essentially the ultimate currency. Lost or stolen time can never be returned or recovered. It reminded me of this post from Wait but Why -- an illustration of the average human lifespan. This hit me hard when I first saw it. Life feels long until you see it laid out in weeks: https://waitbutwhy.com/2014/05/life-weeks.html