asn1_ber_decoder(9)

💖 asn1_ber_decoder — like, the Kernel API Manual, but make it make sense 🎀

okay so bestie, welcome to asn1_ber_decoder(9) — the one from the Kernel API Manual — and honestly? it's giving "quietly iconic little decoder" and I'm obsessed. it decodes BER/DER/CER ASN.1 stuff according to a pattern, and yes we're gonna get into it ✨

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✨ SYNOPSIS ✨

int asn1_ber_decoder (const struct asn1_decoder *decoder , void *context
, const unsigned char *data , size_t datalen );

like. look at her. four args, one int return, main character energy fr fr 💅

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🫶 ARGUMENTS 🫶

• decoder — the decoder definition, and no you don't hand-write it, it's produced by asn1_compiler. your blueprint, your vibe, your whole personality 🦋
• context — the caller's context, and it literally gets passed to the action functions. it's the little backpack your callbacks carry around, iykyk 🎒
• data — the encoded data. the actual bytes. the moment. 🔥
• datalen — the size of the encoded data. gotta know how much byte we're working with, periodt 🌸

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💜 DESCRIPTION 💜

so what it does is: decode BER/DER/CER encoded ASN.1 data according to a bytecode pattern produced by asn1_compiler. that's the whole bit. it reads the bytecode, does the decoding, slays 💅

and the fun part — action functions get called on marked tags, which is how the caller gets to retrieve significant data. so the decoder's like "hey bestie, this tag mattered, here's your moment" and your callback does its thing 🎀

💀 LIMITATIONS 💀 _{(yes I'm saying it dramatically)}

okay so real talk, to keep down the amount of stack used by this function, the following limits have been imposed — and these are actual facts, not vibes, so listen up:

1. this won't handle datalen > 65535 without increasing the size of the cons stack elements and length_too_long checking. so like, don't come for her with a giant blob and expect magic 🙈
2. the stack of constructed types is 10 deep. if the depth of non-leaf constructed types exceeds this, the decode will fail. ten. that's the number. respect it 😭
3. the SET type (not the SET OF type) isn't really supported — because tracking what members of the set have been seen is a pain. that's it, that's the reason, and honestly? fair. 💀

ngl these limits are load-bearing. hit them and the decode fails, no cap. plan accordingly bestie 🫶

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🌈 SEE ALSO 🌈

• Kernel file ./lib/asn1_decoder.c — go read the source, it's right there, iykyk ⭐

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and that's the page! 💖 this little gem lives in ./lib, brought to you by the October 2026 crew — mwah, thanks for the kernel docs, you ate 👑💅✨