check_buffer(9)

✨ check_buffer — the vibe check for buffer safety ✨

Okay bestie, so picture this: it's check_buffer(9), a kernel API manual page (yeah, the 9 means section 9, kernel developer land 😌), and it's basically the bouncer at the club door of memory access. It checks if a buffer and an offset with a byte count are even safe to touch. No cap, this is important stuff. Let me break it down for you 💅

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💖 SYNOPSIS

bool check_buffer (struct binder_alloc *alloc , struct binder_buffer
*buffer , binder_size_t offset , size_t bytes );

That's the whole vibe: takes some binder allocator info, a buffer, an offset into the buffer's data, and a byte count, then hands you back a bool. Slay.

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🌸 ARGUMENTS

• alloc — the binder_alloc for this proc. Lowkey the home base for everything buffer-related.
• buffer — the binder buffer that's getting accessed. The main character, honestly.
• offset — where in the buffer data you're looking. An offset into the buffer data, bestie.
• bytes — how many bytes you're trying to touch starting from that offset.

💀 DESCRIPTION

So the actual job of this function is to verify that the offset/bytes pair fits inside the given buffer's size, and also that the buffer is currently active and not freeable. Two firm rules of the house:

1. The buffer is active — like, actually active — not just sitting there.
2. The offsets have to be multiples of sizeof(u32). The kernel has standards, okay? 🎀

The kernel is allowed to touch the buffer in exactly two situations:

1. When the buffer is being created — that means (buffer->free == 0 && buffer->allow_user_free == 0).
2. When the buffer is being torn down — that means (buffer->free == 0 && buffer->transaction == NULL).

Outside of those two scenarios, it's a no. Nada. Not today. 🛑

✨ RETURN

Returns true if the buffer is safe to access. Otherwise it's giving false, and you should not touch that buffer, periodt.

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👑 SEE ALSO

The kernel file lives at ./drivers/android/binder_alloc.c — that's the source, bestie.

Related functions, all in the binder_alloc family, all iconic in their own right:

• binder_alloc_prepare_to_free(9)
• binder_alloc_new_buf(9)
• binder_alloc_get_page(9)
• binder_alloc_clear_buf(9)
• binder_alloc_free_buf(9)
• binder_alloc_mmap_handler(9)
• binder_alloc_print_allocated(9)
• binder_alloc_print_pages(9)
• binder_alloc_get_allocated_count(9)
• binder_alloc_vma_close(9)
• binder_alloc_free_page(9)
• binder_alloc_init(9)
• binder_alloc_copy_user_to_buffer(9)

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That's the tea 🍓 Formatting note for the very end: this page ships with the Linux kernel v6.1 manual set, dated October 2026, and the source lives under ./drivers/android. So if anyone asks where this came from, that's the whole story. Byeee 🫶💜