Ciro Santilli @cirosantilli 37

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

Section type: sh_type == SHT_RELA.

Common name: "relocation section".

.rela.text holds relocation data which says how the address should be modified when the final executable is linked. This points to bytes of the text area that must be modified when linking happens to point to the correct memory locations.

Basically, it translates the object text containing the placeholder 0x0 address:

   a:       48 be 00 00 00 00 00    movabs $0x0,%rsi
  11:       00 00 00

to the actual executable code containing the final 0x6000d8:

4000ba: 48 be d8 00 60 00 00    movabs $0x6000d8,%rsi
4000c1: 00 00 00

It was pointed to by sh_info = 6 of the .symtab section.

readelf -r hello_world.o outputs:

Relocation section '.rela.text' at offset 0x3b0 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
00000000000c  000200000001 R_X86_64_64       0000000000000000 .data + 0

The section does not exist in the executable.

The actual bytes are:

00000370  0c 00 00 00 00 00 00 00  01 00 00 00 02 00 00 00  |................|
00000380  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

The struct represented is:

typedef struct {
    Elf64_Addr  r_offset;
    Elf64_Xword r_info;
    Elf64_Sxword    r_addend;
} Elf64_Rela;

So:

370 0: r_offset = 0xC: address into the .text whose address this relocation will modify
370 8: r_info = 0x200000001. Contains 2 fields:
- ELF64_R_TYPE = 0x1: meaning depends on the exact architecture.
- ELF64_R_SYM = 0x2: index of the section to which the address points, so .data which is at index 2.
The AMD64 ABI says that type 1 is called R_X86_64_64 and that it represents the operation S + A where:
- S: the value of the symbol on the object file, here 0 because we point to the 00 00 00 00 00 00 00 00 of movabs $0x0,%rsi
- A: the addend, present in field r_added
This address is added to the section on which the relocation operates.
This relocation operation acts on a total 8 bytes.
380 0: r_addend = 0

So in our example we conclude that the new address will be: S + A = .data + 0, and thus the first thing in the data section.

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ELF Hello World Tutorial / Implementations by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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Compiler toolchains generate and read ELF files.
Sane compilers should use a separate standalone library to do the dirty work. E.g., Binutils uses BFD (in-tree and canonical source).

Operating systems read and run ELF files.
Kernels cannot link to a library nor use the C stlib, so they are more likely to implement it themselves.
This is the case of the Linux kernel 4.2 which implements it in th file fs/binfmt_elf.c.

Specialized libraries. Examples:
- github.com/eliben/pyelftools. By a hardcore Googler: plus.google.com/+EliBenderskyGplus/posts
- sourceforge.net/projects/elftoolchain

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Davisson-Germer experiment by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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Ultraviolet by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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E. Coli Whole Cell Model by Covert Lab / Other run variants by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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Besides time series run variants, conditions can also be selected directly without a time series as in:

python runscripts/manual/runSim.py --variant condition 1 1

which select row indices from reconstruction/ecoli/flat/condition/condition_defs.tsv. The above 1 1 would mean the second line of that file which starts with:

"condition" "nutrients" "genotype perturbations" "doubling time (units.min)" "active TFs"
"basal" "minimal" {} 44.0 []
"no_oxygen" "minimal_minus_oxygen" {} 100.0 []
"with_aa" "minimal_plus_amino_acids" {} 25.0 ["CPLX-125", "MONOMER0-162", "CPLX0-7671", "CPLX0-228", "MONOMER0-155"]

so 1 means no_oxygen.

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Mathematical, Physical and Life Sciences division of the University of Oxford by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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www.mpls.ox.ac.uk/

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Snap (package manager) by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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merlijn.sebrechts.be/blog/2020-08-02-why-one-snap-store/ has some very good comments on how snap is more closed than Flatpak.

Snap's permission system is extremely annoying, notably restricting access to home files. They need a popup that says "permission required by app, accept?" urgently!!!

askubuntu.com/questions/1238211/how-to-make-snaps-access-hidden-files-and-folders-in-home

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Cool data embedded in the Bitcoin blockchain / cryptograffiti.info by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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cryptograffiti.info

github.com/1Hyena/cryptograffiti

twitter.com/cryptograffiti (marked as joined March 2014)

Bitcoin blockchain image indexer and uploader. Uses fake P2PKH address.

At some point it stopped using Bitcoin mainline and moved to Bitcoin Cash instead: www.newsbtc.com/news/bitcoin/cryptograffiti-rejects-bitcoin-core-bch-now-available-payment-method/ and therefore became useless. Existing indexes seem to have been broken as well.

Also, based on the timing of Figure "Erich Erstu", this service may be responsible for a large part of the raw JPEG images present in the blockchain from block 416527 (2016) onwards. This is also suggested by the comments at Figure "Tank Man".

A Quantitative Analysis of the Impact of Arbitrary Blockchain Content on Bitcoin gives the interesting insight that all its transactions seem to return change/fees to one or two given addresses, thus making it very easy to list all their uploads if they were consistent! So all we need are some starting points, which we have mostly due to ASCII mentions of the site on known inscriptions, all of which have a few common spent addresses at the very end:

4c903a377addab7c1e35a685d3dabc664199e406374b1e5ce2fc59e78fb5b754: 1MVpQJA7FtcDrwKC6zATkZvZcxqma4JixS
87aad85c6cd75a516789f364637d243c668e3424d031ae510e43c6edfe6ed206: 1MVpQJA7FtcDrwKC6zATkZvZcxqma4JixS
c206e8fff656f07b27dac831ef9b956792bae4e76a2cb43f14f49f0298bf2c2f: 1MVpQJA7FtcDrwKC6zATkZvZcxqma4JixS
ca4f11131eca6b4d61daf707a470cfccd1ef3d80a6f8b70f1f07616b451ca64e: 1MVpQJA7FtcDrwKC6zATkZvZcxqma4JixS

so we just have to solve get all Bitcoin transactions from and to a given address and we are done. Blockchair shows about 800 entries as of February 2024, between 4f94f97eb156b8563a213bb292314a0bd9c95b39afc521fc5965d050daab2a78 (2014-03-02) and ac5f4ea03597b43a72fb8ab42bd5384629f87f4f4abc534f38b8c15148ccaf9f (2017-10-12): blockchair.com/bitcoin/outputs?s=time(desc)&q=recipient(1MVpQJA7FtcDrwKC6zATkZvZcxqma4JixS)

Other related transactions:

tx 87aad85c6cd75a516789f364637d243c668e3424d031ae510e43c6edfe6ed206 block 474652 (2017-07-07) via cryptograffiti.info the default pandoc markdown pandoc.org/try markdown tutorial string! First, unseen in our ASCII dumps due to UTF-8 encoding::
```
Unicode test: `Ä Ö Õ Ü ä ö õ ü`.
```
followed by:
```
An h1 header
============

Paragraphs are separated by a blank line.
```
And if ends with:
```
Uploaded from http://cryptograffiti.info to demonstrate Markdown rendering.
```

TODO understand what these are:
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Clear client-side storage on Chromium by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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Classification of 6-transitive groups by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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math.stackexchange.com/questions/700235/is-there-an-easy-proof-for-the-classification-of-6-transitive-finite-groups says there aren't any non-boring ones.

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BusyBox by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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How to convert async to sync in JavaScript by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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God, it's impossible! You just have to convert the entire fucking call stack all the way up to async functions. It could mean refactoring hundreds of functions.

To be fair, there is a logic to this, if you put yourself within the crappiness of the JavaScript threading model. And Python is not that much better with its Global Interpreter Lock.

The problem is that async was introduced relatively late, previously we just had to use infinitely deep callback trees, which was worse:

myAsync().then(ret => myAsync2(ret).then(ret2 => myAsync3(re3)))

compared to the new infinitely more readable:

ret = await myAsync()
ret2 = await myAsync2(ret)
ret3 = await myAsync3(ret3)

But now we are in an endless period of transition between both worlds.

It is also worth mentioning that callbacks are still inescapable if you really want to fan out into a non-linear dependency graph, usually with Promise.all:

await Promise.all([
  myAsync(1).then(ret => myAsync2(ret)),
  myAsync(2).then(ret => myAsync2(ret)),
])

Bibliography:

And then, after many many hours of this work, you might notice that the new code is way, way way slower than before, because making small functions async has a large performance impact: madelinemiller.dev/blog/javascript-promise-overhead/. Real world case with a 4x slowdown: github.com/ourbigbook/ourbigbook/tree/async-slow.

Anyways, since you Googled here, you might as well learn the standard pattern to convert callbacks functions into async functions using a promise: stackoverflow.com/questions/4708787/get-password-from-input-using-node-js/71868483#71868483

Figure 1.
async function Teletubbies meme
. Source. TODO find original source.

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F-Droid by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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Annapurna Labs by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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Page contains a good summary of their hardware to date. They seem to still be the centerpiece of silicon development. There are still however people outside of Israel doing it, e.g.: www.linkedin.com/in/laurasharpless/ says as of 2021:

My team develops software for our next-generation Machine Learning accelerators: HAL, firmware, and SoC models.

2021: networking chip reports emerge: www.theverge.com/circuitbreaker/2021/3/30/22358633/amazon-reportedly-custom-network-switch-silicon-aws, presumably contesting with the likes of Cisco?

2018 onwards: Amazon AI accelerator silicon.

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x86 Paging Tutorial / Intel manual by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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Although it is impossible to understand without examples in mind, try to get familiar with the manuals as soon as possible.

Intel describes paging in the Intel Manual Volume 3 System Programming Guide - 325384-056US September 2015 Chapter 4 "Paging".

Specially interesting is Figure 4-4 "Formats of CR3 and Paging-Structure Entries with 32-Bit Paging", which gives the key data structures.

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x86 Paging Tutorial / Example: multi-level paging scheme by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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x86 Paging Tutorial / TLB by

Ciro Santilli 37  Updated 2025-07-01  +Created 1970-01-01

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The Translation Lookahead Buffer (TLB) is a cache for paging addresses.

Since it is a cache, it shares many of the design issues of the CPU cache, such as associativity level.

This section shall describe a simplified fully associative TLB with 4 single address entries. Note that like other caches, real TLBs are not usually fully associative.

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x86 Paging Tutorial / Bibliography by