[nbromberg-dn]

The issue

When passing --vendclass '<EN> <DATA>' via CLI arguments, the packet itself is sent with empty data (only with the EN).

The vendclass parser split the enterprise number from its class data by writing a NUL into optarg.
The command-line options are applied more than once: first while processing global options, then again when configuring each interface.

That made the first pass succeed because it kept a local pointer to the bytes after the separator, but it permanently shortened the shared argv string to the EN for later passes.

Example

Calling

$ dhcpcd -6 --vendclass "1234 Hello" ...

Transmitted on the wire (dissected using tshark):

    Vendor Class
        Option: Vendor Class (16)
        Length: 6
        Enterprise ID: Linkage Software Inc. (1234)
        vendor-class-data:

Live-debugging with GDB confirms:

Breakpoint 2, parse_option (ctx=ctx@entry=0x7fffffffd978, ifname=ifname@entry=0x0, ifo=ifo@entry=0x56c940, opt=147, arg=0x56a6a0 "1234", ldop=ldop@entry=0x0, edop=0x0) at if-options.c:2198
2198			u = (uint32_t)strtou(arg, NULL, 0, 0, UINT32_MAX, &e);


(gdb) x/10bc arg
0x56a6a0:	49 '1'	50 '2'	51 '3'	52 '4'	0 '\000'	72 'H'	101 'e'	108 'l'
0x56a6a8:	108 'l'	111 'o'

(gdb) p arg
$1 = 0x56a6a0 "1234"

(gdb) p fp
$2 = 0x56a6a5 "Hello"

The proposed fix

Parse the enterprise number from a temporary NUL-terminated copy instead, leaving argv intact so the per-interface pass can still see and encode the vendor-class data.
fp still advances via strskipwhite into the vendor-class portion - behavior matches the old in-place split without mutating shared optarg.

With this fix, calling the same dhcpcd command as above now trasmits:

    Vendor Class
        Option: Vendor Class (16)
        Length: 11
        Enterprise ID: Linkage Software Inc. (1234)
        vendor-class-data: Hello

And GDB showing:

(gdb) x/10bc arg
0x5824e0:	49 '1'	50 '2'	51 '3'	52 '4'	32 ' '	72 'H'	101 'e'	108 'l'
0x5824e8:	108 'l'	111 'o'
# The 5th byte is kept as whitespace rather than changed to \0 as before

(gdb) p bp
$1 = 0x5977a0 "1234"
(gdb) p arg
$2 = 0x5824e0 "1234 Hello"

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• src/if-options.c

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Walkthrough

The vendor class option parser copies the enterprise-number prefix before parsing it. It leaves the original argument unchanged for later per-interface option replay.

Changes

Vendor class parsing

Layer / File(s)	Summary
Non-destructive enterprise-number parsing src/if-options.c	The parser copies the enterprise-number prefix into a temporary buffer, handles allocation failure, parses the copied value, and frees the buffer. The original argument remains unchanged.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: colinmcinnes

Merge Risk: ⚪ Minimal · up to eb70f

No actionable merge risk was identified in the vendor-class argument preservation change.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly identifies the main change: preserving the IPv6 vendclass command-line argument while parsing its enterprise number.
Description check		The description directly explains the vendclass parsing bug, its cause, the proposed fix, and the resulting behavior.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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[nbromberg-dn]

Thanks @ColinMcInnes for the approval.
@rsmarples may you please take a look?