Fix Position and Seek for MemoryStream with custom origin

[manandre]

Fixes#88541

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Issue Details

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Fixes #88541

Author:	manandre
Assignees:	-
Labels:	area-System.IO
Milestone:	-

[danmoseley]

these last two cases are causing OOM on 32 bit

https://helixre107v0xdcypoyl9e7f.blob.core.windows.net/dotnet-runtime-refs-pull-88572-merge-044351d2f9494dfeb8/System.IO.Tests/1/console.f70c368a.log?helixlogtype=result

[elgonzo]

Note that this will change the behavior of the MemoryStream.Property and will be in conflict with the current MemoryStream.Position documentation (https://learn.microsoft.com/en-us/dotnet/api/system.io.memorystream.position?view=net-8.0#exceptions). If this pull request is accepted, please don't forget to update the MemoryStream.Position documentation accordingly.

[jozkee]

@elgonzo@manandre I do believe this is the right approach and we should continue with updating the docs for Seek() and Position accordingly.

[jozkee]

Can you please test with an offset of long.MinValue + 1 (uncasted). The check we discussed in https://github.com/dotnet/runtime/pull/88572/files/6880c4d6d148f46a878f929303fd2472e8fbc363#r1297747637 may be there to prevent an overflow after the (int) conversion.

[elgonzo]

Yes, removing the unchecked(loc + offset) < _origin check was a mistake. Because (int)offset used for calculating tempPosition will just cut the upper 32 bit from the long value, turning certain negative offset values into positive tempPosition values that are equal to or larger than _origin, thus not correctly failing on offset values representing invalid negative positions. For example, with this change, calling Seek(long.MinValue, SeekOrigin.Begin) would lead to tempPosition being equal to the value of _origin and effectively set the stream position to zero instead of throwing, if i am not mistaken.

[manandre]

I have added two Seek calls with long.MinValue + 1 and long.MaxValue - 1 as offset and checked that an exception is correctly raised in all cases.
Indeed I have manually checked that these calls are not always sending an exception when the unchecked(loc + offset) < _origin check is not present.

[jozkee]

LGTM, thanks!