Semihosting SYS_READ returns incorrect value on EOF

[donmr]

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• Programmer/board type: e.g Stlink/v2-1 on STM32F407-DISC (updated to latest).
• Programmer firmware version: e.g STSW-LINK007 V2J30M21
• Operating system: Linux-Mint 18.3
• Stlink tools version and/or git commit hash: v1.4.0-39-g6db0fc2
• Stlink commandline tool name: st-util
• Target chip (and optional board): STM32F407VGT6U

A as-detailed description possible of the problem with debug output when available.

When SYS_READ is called at the end of a file, it is returning a negative value (-len). This does not match the ARM spec and confuses the stdio code on the DUT.

I believe the issue is near line 353 of src/gdbserver/semihosting.c

Output:

2018-07-25T07:18:13 DEBUG semihosting.c: Semihosting: read(14, target_addr:0x2001ff60, 16)
2018-07-25T07:18:13 DEBUG semihosting.c: Semihosting: read 0 bytes
2018-07-25T07:18:13 DEBUG common.c: *** stlink_write_mem32 0 bytes to 0x2001ff60
2018-07-25T07:18:13 DEBUG semihosting.c: Semihosting: return -16

Expected/description:

Per ARM spec, return value should be the same as the requested read length.

[donmr]

I believe that changing line 353 from:
*ret -= buffer_len;
To:
*ret = buffer_len - *ret;
Fixes this problem. I don't have an exhaustive test suite, but it works for full, partial, and empty reads.

[xor-gate]

Yeah potential this is a bug at https://github.com/texane/stlink/blob/master/src/gdbserver/semihosting.c#L353. Ping @Fabien-Chouteau not sure what is happening here.

[donmr]

Just a side note, I don't like the way this code stores the result of the read() call into *ret because that value is not the proper return value, ever. I think it would be cleaner to use a local variable for that. Then the computation of the return code would be more clear.

[xor-gate]

Thanks for the fix in #727.

[Fabien-Chouteau]

I agree with the change but I think the patch that was merged is fault as well. See my review.